[clang] clang/AMDGPU: Merge toolchain subclasses (PR #204863)

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hongtaihu wrote:

 I can no longer reproduce #199543 with current trunk. The original crash was in HIPAMDToolChain::addClangTargetOptions, which was removed by this refactor. Is it likely that #204863 fixed the issue?

https://github.com/llvm/llvm-project/pull/204863