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

[Joseph Huber via cfe-commits]

jhuber6 wrote:

> We should not have to build LLVM with clang to be able to build libclc. At least that was not a constaint before #185247

This was always a constraint, the difference is that `libclc` before that PR would grab `clang/` from your environment and use it via custom commands. That PR expects users to configure the compiler.

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