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

[Romaric Jodin via cfe-commits]

rjodinchr wrote:

> 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.

I disagree.

Before, the top-level CMake made sure to build clang (it still does) and all the tools needed.
But right now something is missing to tell the external-project (libclc) to use those tools that have just been built for that purpose.

When we build libclc as a subproject of LLVM, users expect LLVM CMake to configure libclc properly. It should not be their responsibility to change how they build LLVM to accommodate libclc.

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