Currently implicit returns do not correctly resolve the async generator
objects. This is observable via AsyncGenerator#throw as the implicit
return won't override the rejection.

Bug: v8:10238
Change-Id: I012fc3507d1e4106e7f35b21275be180a6e274c3
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2065343Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Shu-yu Guo <syg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66413}