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

[Wenju He via cfe-commits]

wenju-he wrote:

> If you're curious, this is what it looks like for the offloading toolchain, this one enables pretty much every runtime supported for the GPU except `libclc`. https://github.com/llvm/llvm-project/blob/main/offload/cmake/caches/FlangOffload.cmake

thanks, it is very helpful

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