Design doc:
https://docs.google.com/document/d/1kL08cz4lR6gO5b2FATNK3QAfS8t-6K6kdk88U-n8tug/edit

This CL is a follow-up after the original implementation, see CL:
https://chromium-review.googlesource.com/c/v8/v8/+/1106977

It includes a fix for the missing async generators optimization,
as well as cleanup of the manual patching of the builtins. It also includes
mjsunit test for all usages of the new behaviour.

Bug: v8:8267

Change-Id: I999f341acb746c6da5216e44b68a519656fd5403
Reviewed-on: https://chromium-review.googlesource.com/c/1261124Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Maya Lekova <mslekova@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56414}