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

[via cfe-commits]

h-vetinari wrote:

If you want to backport this, you need to add the PR to the milestone and comment 

`/cherry-pick <hash1> <hash2> <hash3> ...`

Then the bot should open a PR (or it'll tell you if it failed to cherry-pick, and how to proceed from there).

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