meson: simplify libpolly handling

since we don't require it explicitly, we're doing too much work, just
put it in the dep_clang list, whether it's found or not doesn't matter.
This also avoids doing a lookup for polly if we find clang via cmake.

CC: 20.3 21.0 <mesa-stable@lists.freedesktop.org>
91 jobs for !8479 with submit/use-cmake-to-find-clang in 8 seconds (queued for 4 seconds)
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Status Job ID Name Coverage
  Sanity
passed #6637235
sanity

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  Container
manual #6637238
aarch64 manual
arm_build
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manual
arm_test-base
manual #6637240
windows shell 1809 mesa manual
windows_build_vs2019
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manual
x86_build-base
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manual
x86_test-base
 
  Container 2
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android_build
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arm64_test
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armhf_test
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i386_build
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kernel+rootfs_amd64
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aarch64
kernel+rootfs_arm64
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aarch64
kernel+rootfs_armhf
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ppc64el_build
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s390x_build
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x86_build
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x86_test-gl
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x86_test-vk
 
  Meson X86 64
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meson-clang
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meson-clover
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meson-clover-testing
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meson-gallium
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meson-release
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meson-testing
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meson-testing-asan
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meson-vulkan
 
  Build Misc
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meson-android
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aarch64
meson-arm64
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aarch64
meson-arm64-build-test
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aarch64
meson-armhf
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meson-i386
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meson-mingw32-x86_64
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kvm
meson-ppc64el
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kvm
meson-s390x
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windows docker 1809 mesa
meson-windows-vs2019
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allowed to fail
scons-win64
 
  Amd
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mesa-ci-aarch64-lava-collabora
radeonsi-stoney-gles2:amd64
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mesa-ci-aarch64-lava-collabora
radeonsi-stoney-gles31:amd64
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mesa-ci-aarch64-lava-collabora
radeonsi-stoney-gles3:amd64
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mesa-ci-aarch64-lava-collabora
radeonsi-stoney-traces:amd64
 
  Arm
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mesa-ci-aarch64-lava-collabora
panfrost-g52-gles2:arm64
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mesa-ci-aarch64-lava-collabora
panfrost-g52-gles3:arm64
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mesa-ci-aarch64-lava-collabora
panfrost-t720-gles2:arm64
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mesa-ci-aarch64-lava-collabora
panfrost-t860-gles2:arm64
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mesa-ci-aarch64-lava-collabora
panfrost-t860-gles3:arm64 1/2
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mesa-ci-aarch64-lava-collabora
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mesa-ci-aarch64-lava-collabora
panfrost-t860-traces:arm64
 
  Freedreno
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google-freedreno-db410c
arm64_a306_gles2
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google-freedreno-db410c
arm64_a306_gles3
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google-freedreno-db410c
arm64_a306_gles3_options
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google-freedreno-db820c
arm64_a530_gles2 1/2
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google-freedreno-db820c
arm64_a530_gles2 2/2
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google-freedreno-db820c
arm64_a530_gles3
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google-freedreno-db820c
arm64_a530_gles31
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google-freedreno-db820c
arm64_a530_piglit_shader
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google-freedreno-cheza
arm64_a630_gl
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google-freedreno-cheza
arm64_a630_gles2
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google-freedreno-cheza
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google-freedreno-cheza
arm64_a630_gles31
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google-freedreno-cheza
arm64_a630_gles_others
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google-freedreno-cheza
arm64_a630_piglit_shader
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google-freedreno-cheza
arm64-a630-traces
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google-freedreno-cheza
arm64_a630_vk 1/2
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google-freedreno-cheza
arm64_a630_vk 2/2
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google-freedreno-cheza
arm64_a630_vk_sysmem
 
  Raspberrypi
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igalia-rpi3
vc4-rpi3-gles2:armhf 1/4
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igalia-rpi3
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igalia-rpi3
vc4-rpi3-gles2:armhf 3/4
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igalia-rpi3
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  Software Renderer
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lavapipe-vk
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llvmpipe-gles2
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llvmpipe-piglit-cl
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llvmpipe-piglit-glslparser
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llvmpipe-piglit-quick_gl
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llvmpipe-piglit-quick_shader
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llvmpipe-traces
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softpipe-gl
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softpipe-gles2
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softpipe-gles3
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softpipe-gles31 1/2
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softpipe-gles31 2/2
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softpipe-piglit-quick
 
  Layered Backends
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windows docker 1809 mesa
test-d3d12-windows
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virgl-gl30-on-gl
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virgl-gl31-on-gl
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virgl-gl32-on-gl
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virgl-gles2-on-gl
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virgl-gles31-on-gl 1/2
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virgl-gles31-on-gl 2/2
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virgl-gles3-on-gl
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virgl-traces