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

[Chandler Carruth via cfe-commits]

chandlerc 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: [dropbox.com/scl/fi/d6e334heik4wp2dlztlq8/log_test.35a7fd79c471cdebd416ba711dbae6970e0e8550.zip?rlkey=kdkeo4897mw3642f9jyn4cmd0&dl=0](https://www.dropbox.com/scl/fi/d6e334heik4wp2dlztlq8/log_test.35a7fd79c471cdebd416ba711dbae6970e0e8550.zip?rlkey=kdkeo4897mw3642f9jyn4cmd0&dl=0)

Hmm, this looks like I just didn't fix "enough".

I've sent out #121043 and rebased this PR on top of that as well. Can you take another spin?

This PR should be the right branch, incorporating all the other changes.

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