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

[Chandler Carruth via cfe-commits]

chandlerc wrote:

> > I think I've addressed most of the review comments here at this point.
> > But maybe most excitingly, I think the latest version may dodge the issues that have cropped up with MSVC -- both LoongArch and X86 fixes have been incorporated that hopefully help. @dyung -- if you can try it out, let me know.
> 
> 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.

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