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

[Chandler Carruth via cfe-commits]

chandlerc wrote:

I know it's only been a few days, but pinging in the hope of landing this week... This seems to finally be in a good state and is somewhat hard to keep rebasing. Happy to do anything I can to help make review easier.

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