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

[Joseph Huber via cfe-commits]

jhuber6 wrote:

Correct me if I'm worng, but hasn't `libclc` always required the compiler to be clang? I don't think MSVC supports OpenCL.

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