[clang] [coroutines] Introduce [[clang::coro_lifetimebound]] (PR #72851)

[Ilya Biryukov via cfe-commits]

https://github.com/ilya-biryukov approved this pull request.

LGTM with a few suggestiosn.

One is in the comment.
I also suggest to add a warning when users mark the type as `coro_lifetimebound` without `coro_return_type`.
The analysis will be disabled in this case and it may be hard to understand why.

However, I don't think that's a reason to block this change. This could be a follow-up.

https://github.com/llvm/llvm-project/pull/72851