[C++20] [Coroutines] Mark coroutine done if unhandled_exception throws

According to [dcl.fct.def.coroutine]/p14:
> If the evaluation of the expression promise.unhandled_­exception()
> exits via an exception, the coroutine is considered suspended at the
> final suspend point.

But this is not implemented in clang before. This patch would implement
this feature by marking the coroutine as done at the place of
coro.end(frame, /*InUnwindPath=*/true ).

Reviewed By: rjmccall

Differential Revision: https://reviews.llvm.org/D115219