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

[Matt Arsenault via cfe-commits]

arsenm 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.

Yes and no. There were always 2 compilers. The host complier was still used for building utils, like prepare-builtins. It would then find clang and invoke it as a separate tool 

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