Allocates the Await success/failure closures, their context, and
the two required JSPromise objects all at once in a single call,
rather than performing multiple allocations throughout the function.

Saves about 2kb of snapshot space on an x64.release build.

Performance impact of this change has not been measured yet.

BUG=v8:4483
R=ishell@chromium.org, jgruber@chromium.org

Change-Id: I8d911cb91f5d0e00544ad3ba608aa170f6b2f704
Reviewed-on: https://chromium-review.googlesource.com/549999Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Caitlin Potter <caitp@igalia.com>
Cr-Commit-Position: refs/heads/master@{#46360}