[libclc] [llvm] [libclc][CMake] Use clang/llvm-ar on Windows (PR #186726)

[Wenju He via llvm-commits]

wenju-he wrote:

> Probably fine for now, but long term I'd recommend we consider moving everything to the interface the other GPU runtimes use. It's definitely much more annoying for the user however, but that's what CMake cache files are for.

Thanks, that is great suggestion. I'll follow up.

https://github.com/llvm/llvm-project/pull/186726