[compiler-rt] [libclc] [libcxx] [libcxxabi] [libunwind] [llvm] [CMake] Remove the use of AddLLVM from runtimes (PR #217502)

[Louis Dionne via cfe-commits]

https://github.com/ldionne approved this pull request.

I think this is a great step to clean up the CMake setup for the runtimes.

LGTM, looks like there are a few issues still being worked out but assuming they are resolved, I think this is 100% the right direction.

https://github.com/llvm/llvm-project/pull/217502