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

[via cfe-commits]

dyung wrote:

> No worries about delay, this gives me a credible target to resolve the rest of the issues. I'll update this PR both to address review comments but also to try and address the rest of the failures. Appreciate runs to validate these updates. =]

Definitely happy to help out with runs as we do not want our stuff broken internally! Just ping me and I'd be happy to run your changes on our build bot.

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