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

[Yaxun Liu via cfe-commits]

yxsamliu wrote:

I feel choosing toolchain based on input files does not solve all the use cases.

You may want to handle the object files, bitcodes, or assembly files differently by using different toolchains, e.g. you may want to choose rocm toolchain or amdgpu toolchain or HIPAMD toolchain to hand the object file, even though it has the same extension. Although you could introduce -x for different object type, e.g. cl_obj, cxx_obj, or hip_obj, but that is quite cubbersome.

I am wondering whether we should introduce a clang option which allows us to directly choose a toolchain.

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