This extends the Promise#then protector to also guard the intrinsic
%ObjectPrototype%, making it usable for fast-path logic in the
AsyncGeneratorResolve operation, where we can skip the "then" lookup
on the iteratorResult in that case. We also add a corresponding fast-
path to the ResolvePromise builtin itself, which avoids the second
"then" lookup on the async iterator side.

This reduces execution time of the fibonacci-async-es2017-native test
from 186.9ms to 171.2ms, which corresponds to an improvement of 8.4%
in this case.

Bug: v8:7253
Change-Id: Ia09effc81a016703780d5c308f541e3d797ccabe
Reviewed-on: https://chromium-review.googlesource.com/967203Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52024}