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

[via cfe-commits]

dyung wrote:

> > Sure, I can test it. Just to confirm, what branch/commit should I be testing?
> 
> This PR has everything in it so you can just test it. There are 3 commits on this branch that won't land here (they're under review in their own PRs), but I've got them all here so you can just test this PR.

Unfortunely it still looks like there are a bunch of failures. They do look different from before though. I've uploaded the test log here: https://www.dropbox.com/scl/fi/d6e334heik4wp2dlztlq8/log_test.35a7fd79c471cdebd416ba711dbae6970e0e8550.zip?rlkey=kdkeo4897mw3642f9jyn4cmd0&dl=0

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