meson: try to use cmake as a finder for clang

Clang (like LLVM), very annoyingly refuses to provide pkg-config, and
only provides cmake (unlike LLVM which at least provides llvm-config,
even if llvm-config is terrible). Meson has gained the ability to use
cmake to find dependencies, and can successfully find Clang. This change
attempts to use cmake to find clang instead of a bunch of library
searches, when paired with -Dcmake_prefix_path we can much more reliably
use cmake to control which clang we're getting. This is only enabled for
meson >= 0.51, which adds the required options.

Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
10 jobs for !936 with llvm-cmake in 17 minutes and 44 seconds (queued for 59 seconds)
detached
Status Job ID Name Coverage
  Containers Build
passed debian #385998

00:00:23

 
  Build+Test
passed meson-clang #386000

00:10:05

passed meson-clover #386004

00:09:11

passed meson-main #386003

00:07:43

passed meson-swr-glvnd #385999

00:08:18

passed meson-vulkan #386005

00:02:08

passed scons-llvm #386007

00:03:49

passed scons-nollvm #386006

00:04:34

passed scons-swr #386001

00:04:49

passed scons-win64 #386002

00:06:35