[clang] [AMDGPU] Use the AMDGPUToolChain when targeting C/C++ directly (PR #99687)

[Jon Chesterfield via cfe-commits]

https://github.com/JonChesterfield approved this pull request.

This is the right thing. amdgpu is completely usable without rocm libraries and already has out of tree users doing that. Needing to manually opt out of rocm libs when not using any of rocm is definitely annoying, especially in contexts like godbolt where one is just trying to look at the emitted asm.

The code is ugly but that's necessary to preserve opencl's behaviour, agreed that it's not worth trying to change that.

https://github.com/llvm/llvm-project/pull/99687