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

[Chandler Carruth via cfe-commits]

chandlerc wrote:

Thanks @rnk !

I've fixed the one style nit (doh!) and a few surrounding variables.

I'm working on rebasing everything now.

But one specific question: would you prefer me to land as a series of commits or a single squashed commit for the entire PR? I'm happy either way. My mild preference is to prefer the series of commits, but open to suggestions here.

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