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

[Romaric Jodin via cfe-commits]

rjodinchr wrote:

the CMake invocation is directly on clspv CMake.

In clspv CMakeLists.txt it uses `add_subdirectory` to pull llvm as a subproject (not external).
clspv defines the expected defines and leave the rest to llvm's CMake to build what we need, especially libclc.

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