[clang] [coroutines] Introduce [[clang::coro_return_type]] and [[clang::coro_wrapper]] (PR #71945)

[Ilya Biryukov via cfe-commits]

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

LGTM assuming the `getCurFunction()` comment is addressed in some form (moving it to callers or adding an assertion)

Thanks for the change!

https://github.com/llvm/llvm-project/pull/71945