[clang] Patch series to reapply #118734 and substantially improve it (PR #120534)

[Chandler Carruth via cfe-commits]

chandlerc wrote:

@dyung -- this PR is updated with new fixes. NVPTX hopefully works and neither of my debugging checks fires. But there may still be some other failures I need to chase down, let me know?

https://github.com/llvm/llvm-project/pull/120534