[clang] [Clang][Sema] Add -Wredundant-defer to diagnose redundant `_Defer`s (PR #225748)

[Ambrose Leeb via cfe-commits]

https://github.com/Sirraide commented:

This is something we figured would be nice to have when I implemented `defer`, but it wasn’t _that_ important so I didn’t do that for the sake of not blowing up that patch too much, so thanks for working on this!

Overall direction seems fine, but I think there is some refactoring in order here; ideally that would be done in a separate NFC patch first, but I don’t think this patch adds _that_ much so I’m not sure it’s 100% necessary if you don’t want to go through the trouble of doing that.

CC @AaronBallman for opinions on that.

https://github.com/llvm/llvm-project/pull/225748