meson: Add support for using cmake for finding LLVM

meson: Add support for using cmake for finding LLVM

Meson has support for using cmake as a finder for some dependencies,
including LLVM. Using cmake has a lot of advantages: it needs less meson
maintenance to keep working (even for llvm updates); it works more
sanely for cross compiles (as llvm-config is a compiled binary not a
shell script). Meson 0.51.0 also has a new generic variable getter that
can be used to get information from either cmake, pkg-config, or
config-tools dependencies, which is needed for cmake. We continue to
support using llvm-config if you don't have cmake installed, or if cmake
cannot find a suitable version.

Fixes: 0d59459432cf077d768164091318af8fb1612500
       ("meson: Force the use of config-tool for llvm")
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>