This fixes a case in which we forgot to assign flags to TextNodes
created through

AddBmpCharacters
AddNonBmpSurrogatePairs
AddLoneLeadSurrogates
AddLoneTrailSurrogates

functions. If these initially had a flag (e.g. case-insensitive 'i')
set, that information was lost. This bug resulted in missing case
folding in no_i18n builds (perhaps other things as well that just
aren't covered by our test suite).

Cq-Include-Trybots: luci.v8.try:v8_linux_noi18n_rel_ng
Bug: v8:10131,v8:10120
Change-Id: Icef4f0dbd47971a538e07bab2f1067c383fd59c6
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2423718Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70106}