Use the position of commas in async arrow expressions to mark the
initializer position of any parameters that might have been set in the
preceding parameter.

This extends https://chromium-review.googlesource.com/c/v8/v8/+/1710671
to async arrow heads.

Bug: v8:8510, chromium:997320
Change-Id: I98e0ac817c7f53fbf1dced98fb6891a386ee7803
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1781057Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63542}