[llvm] [CMake] enable CMP0144 policy if available (PR #96589)

[Fangrui Song via llvm-commits]

MaskRay wrote:

I use CMake 2.28 and see CMP0144 warnings. This PR will fix that.

However, if I am reading https://cmake.org/cmake/help/latest/policy/CMP0144.html correctly, `rg -g '*CMake*' '_ROOT\b'`, some mlir `<PackageName>_ROOT` variables need to be ported. 

`mlir/lib/Target/LLVM/CMakeLists.txt` @fabianmcg @joker-eph 

https://github.com/llvm/llvm-project/pull/96589