The is_awaiting bit on async generators distinguishes waiting on an
await. When the async generator resumes from an await, the is_awaiting
bit is cleared.

It is possible through overriding Promise#constructor that `await`
throws *after* setting is_awaiting. There is an implicit try-catch
around the body of the async generator such that, usually, caught
exceptions would clear the is_awaiting bit. However, the exception
thrown from a monkeypatched Promise#constructor can be caught by script,
and thus never clear the is_awaiting bit.

This CL sets the is_awaiting bit *after* `await` completes, with the
exception of the return resumption. It is not possible to have the
exception thrown by the await in the return resumption be caught by
script.

Bug: chromium:1171667
Change-Id: I0b615617a5c949f03350ab0f06c42920d43b5488
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2659508Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Shu-yu Guo <syg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72593}