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

[Joseph Huber via cfe-commits]

jhuber6 wrote:

> https://github.com/google/clspv
> 
> We are building a llvm-based compiler, and we define `LLVM_ENABLE_RUNTIMES=libclc`.

I can't really make sense of an entire repository, what does your CMake invocation look like?

The way it works is that for the standard runtimes build you invoke https://github.com/llvm/llvm-project/blob/main/llvm/runtimes/CMakeLists.txt. This creates a sub-project on the `runtimes/` target using LLVMExternalProjectUtils. This passes the USE_TOOLCHAIN option which is intended to tell it to use the just-build clang as the C/CXX compiler. There seems to be logic in here that is MSVC specific that tells it to use the MSVC compatible driver https://github.com/llvm/llvm-project/blob/a70419505471bd8240ef3451dcdd541f8676477c/llvm/cmake/modules/LLVMExternalProjectUtils.cmake#L31.

Can you be more specific in which parts are failing. If you're using it through `cmake ../runtimes` you need to pass the runtime compiler yourself. If you're doing it through `cmake ../llvm` then it should be configured automatically but there is possibly something subtly missing in the CMake there for MSVC.

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