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

[Joseph Huber via cfe-commits]

https://github.com/jhuber6 commented:

I figured this would be solved by the runtimes interface passing `CMAKE_AR` manually. There's the `cmake ../runtimes` case, but if the user is doing this all custom I'd expect it to be easy enough to just override.

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