Always unmark arrowhead parameters as assigned directly after their
initialization as the parser doesn't know when it first sees the
"assignment" that it may be in an arrowhead.

Bug: chromium:1003403, v8:8510
Change-Id: Iad5a4136d5ec06331fc43b81a809fd72cee2dd65
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1815131
Commit-Queue: Dan Elphick <delphick@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63947}