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

[via cfe-commits]

dyung wrote:

``
> 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.

I've built and tested this PR (I think) and it doesn't seem to have changed any from the last run?

Test logs are uploaded here: https://www.dropbox.com/scl/fi/icoi9z54vgayim9mueilg/log_test.979ae6b10db8a74ae3bff7f3f9fd34f535ce0910.zip?rlkey=up5j5kfgxttix4ajlakd1nee4&dl=0

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