[clang] [clang-tools-extra] [Clang] Add support for the C `defer` TS (PR #162848)

[via cfe-commits]

Sirraide wrote:

I’ve been busy w/ expansion statements lately, but I finally managed to track down that codegen bug: essentially, we were clobbering the cleanup slot while emitting cleanups inside the `defer`. I also added a huge comment that explains the situation because it was very much not obvious to me.

https://github.com/llvm/llvm-project/pull/162848