This change also removes the kInvalid value from JSRegExp::Flag, so that
the values in JSRegExp::Flag correspond only to those which can exist on
the heap and not things used temporarily during parsing.

Change-Id: I1ded0b1be8c59eab72320edfef26eda42c91a89f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2216302Reviewed-by: Sathya Gunasekaran  <gsathya@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Seth Brenith <seth.brenith@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#68182}