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

[Martin Storsjö via llvm-commits]

mstorsjo wrote:

> Thank you for your review. The patch works well on my Windows machine. By the way, if this patch is good to go, could you merge it for me?

Sure, will do. I kind of intentionally left the PR unmerged for a little while, after accepting, to give people in other time zones another chance to chime in on it, before merging - but I'll merge it now.

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