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

[via llvm-commits]

c8ef wrote:

Hi @RKSimon, could you please take a look at this patch? I'm unsure why the libc++ test is not running, although it functions correctly locally and in the Buildkite CI.

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