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

[Matt Arsenault via cfe-commits]

https://github.com/arsenm commented:

Does this change the behavior with amdgcn-- or amdgcn-mesa-mesa3d? 

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