clover: Use explicit conversion from llvm::StringRef to std::string

Fixes build after llvm 777180a32b61070a10dd330b4f038bf24e916af1
("[ADT] Make StringRef's std::string conversion operator explicit")

CC: <mesa-stable@lists.freedesktop.org>
Signed-off-by: Jan Vesely <jan.vesely@rutgers.edu>
Reviewed-by: Francisco Jerez <currojerez@riseup.net>
35 jobs for !3635 with fix-llvm-stringref in 17 minutes and 15 seconds (queued for 3 seconds)
detached
Status Job ID Name Coverage
  Container
passed #1535138
aarch64
arm_build

00:00:21

passed #1535139
aarch64
arm_test

00:00:19

passed #1535135
x86_build

00:00:20

passed #1535137
x86_build_old

00:00:20

passed #1535136
x86_test-gl

00:00:21

 
  Build
passed #1535143
aarch64
meson-arm64

00:06:33

passed #1535142
aarch64
meson-armhf

00:07:19

passed #1535144
meson-clang

00:09:00

passed #1535147
meson-clover

00:08:39

passed #1535148
meson-clover-old-llvm

00:06:19

passed #1535150
meson-i386

00:03:48

passed #1535141
meson-main

00:08:39

passed #1535151
meson-mingw32-x86_64

00:06:07

passed #1535140
meson-testing

00:07:06

passed #1535149
meson-vulkan

00:07:12

passed #1535152
scons

00:07:26

passed #1535153
scons-old-llvm

00:04:48

passed #1535145
scons-swr

00:07:39

passed #1535146
scons-win64

00:07:42

 
  Test
passed #1535169
db410c
arm64_a306_gles2

00:07:38

passed #1535157
lava-meson-gxl-s905x-libretech-cc
lima-mali450-test:arm64

00:09:17

passed #1535154
lava-sun50i-h6-pine-h64
panfrost-t720-test:arm64

00:08:19

passed #1535155
lava-rk3288-veyron-jaq
panfrost-t760-test:armhf

00:05:12

passed #1535156
lava-rk3399-gru-kevin
panfrost-t860-test:arm64

00:05:36

passed #1535159
piglit-glslparser

00:04:51

passed #1535158
piglit-quick_gl

00:09:46

passed #1535160
piglit-quick_shader

00:06:22

passed #1535161
test-llvmpipe-gles2

00:03:35

passed #1535162
test-softpipe-gles2

00:02:09

passed #1535165
test-softpipe-gles31 1/4

00:08:47

passed #1535163
test-softpipe-gles3 1/2

00:05:32

passed #1535166
test-softpipe-gles31 2/4

00:04:37

passed #1535167
test-softpipe-gles31 3/4

00:05:26

passed #1535168
test-softpipe-gles31 4/4

00:02:41

passed #1535164
test-softpipe-gles3 2/2

00:05:38