No longer include the "async" keyword, or an async arrow function's single
identifier parameter as part of its inferred name.

BUG=v8:5281, v8:4483
R=adamk@chromium.org, littledan@chromium.org, marja@chromium.org

Review-Url: https://codereview.chromium.org/2235423003
Cr-Commit-Position: refs/heads/master@{#38627}