[llvm] [CMake] enable CMP0147 policy if available (PR #109150)

[Martin Storsjö via llvm-commits]

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

The change seems reasonable to me. AFAIK we have little to no CI setups that would use MSVC with visual studio generators anyway, they all use ninja usually, so any CI errors here should be unrelated failures anyway (and the libc++ CI is having issues lately).

https://github.com/llvm/llvm-project/pull/109150