Also lots of cleanup around the checking for 'await' as an identifier
throughout the parser and preparser.

R=caitp@igalia.com, littledan@chromium.org
BUG=v8:4483,v8:5298

Review-Url: https://codereview.chromium.org/2267493002
Cr-Commit-Position: refs/heads/master@{#38798}