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

[Reid Kleckner via cfe-commits]

rnk wrote:

> But one specific question: would you prefer me to land as a series of commits or a single squashed commit for the entire PR? I'm happy either way. My mild preference is to prefer the series of commits, but open to suggestions here.

I would land it as the six-patch stack in this case. I ended up using that to review, since mainly the first patch is the interesting one and the target-specific migration ones are more mechanical and more incremental.

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