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

[Joseph Huber via cfe-commits]

jhuber6 wrote:

If you're using `add_subdirectory` that's a direct violation of of the runtimes builds are supposed to be used so I'm not surprised it breaks.

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