meson: Error if LLVM is turned off but clover it turned on

Since clover has a hard requirement on LLVM

v2: - make error message more specific
15 jobs for !558 with clover-require-rtti in 17 minutes and 26 seconds (queued for 2 seconds)
latest detached
Status Job ID Name Coverage
  Containers Build
passed #209775
ubuntu

00:01:09

 
  Build+Test
passed #209776
autotools

00:12:41

passed #209778
meson-clang

00:12:15

passed #209784
meson-gallium-clover-llvm

00:06:40

passed #209785
meson-gallium-clover-llvm39

00:06:17

passed #209783
meson-gallium-drivers-other

00:07:34

passed #209782
meson-gallium-radeonsi

00:05:35

passed #209786
meson-gallium-st-other

00:06:35

passed #209777
meson-gallium-swr

00:09:41

passed #209781
meson-glvnd

00:06:39

passed #209780
meson-loader-classic-dri

00:06:00

passed #209779
meson-vulkan

00:04:13

passed #209788
scons-llvm

00:05:38

passed #209787
scons-nollvm

00:05:45

passed #209789
scons-swr

00:08:05