RegExpParser::ParseRegExpSyntax was added to allow the SpiderMonkey
embedding of irregexp to report early errors. It was removed in
https://chromium-review.googlesource.com/c/v8/v8/+/2509596 because it
was unused. This patch restores it, with a comment to prevent future
deletion.

Bug: v8:11368
Change-Id: Iebec7e14b92e9a0fccc08f2f1c85d8ff4d6173f1
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2658037Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72453}