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

[Joseph Huber via cfe-commits]

jhuber6 wrote:

Okay, so the only user of the toolchain is `Darwin`. So I'd need to somehow rework that logic to make it work.

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