[clang] [llvm] [Coroutines] Introduce [[clang::coro_only_destroy_when_complete]] (PR #71014)

[Erich Keane via cfe-commits]

https://github.com/erichkeane approved this pull request.

CFE stuff looks good, but someone more familiar with the LLVM changes needs to do final approval.

https://github.com/llvm/llvm-project/pull/71014