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.

This is WIP because I'm waiting for this meson change to land first.