Change the way that the (internal) await closures store the link to the
generator object by introducing a dedicated AwaitContext, which stores
the generator object into the extension slot (instead of misusing a
regular FunctionContext here). Also unify the allocation+initialization
of these contexts in the await-related builtins (both for async functions
and generators).

The rationale behind this is that for (zero-cost) async stack traces, we
will need to dig into these contexts and we can do better checking with
a dedicated instance type there. As an additional benefit, we save one
word per await context, since we just use (the otherwise unused) extension
slot to remember the generator object. As yet another benefit we will
never accidentally use any of these contexts in the regular scope chain
lookups, meaning we can also catch bugs there. And last but not least
the objects printing machinery understands these contexts now and can
even print the generator object for AwaitContexts for short printing,
which is really valuable for debugging.

Tbr: ulan@chromium.org
Bug: v8:7253, v8:7522, v8:8015
Change-Id: I86955f5701e694e8a10b91ebe5f52705aa90968d
Reviewed-on: https://chromium-review.googlesource.com/1249491Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56301}