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

[Matt Arsenault via cfe-commits]

arsenm 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?

I don't think this was ever that broken (except on macOS hosts, which was fixed by 951c67d4dbed4f87f366f1f911bc061bc6da1912 IIRC)

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