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

[Joseph Huber via cfe-commits]

jhuber6 wrote:

it's also tough to say it's legal to drop the `amdhsa` in this case, because we rely on builtins that refer to things like implicit arguments, which are from HSA. Otherwise I would probably be fine just calling it `amdgcn`.

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