This flag allows invalid escape sequences in tagged templates, which is
a stage-4 TC39 proposal shipping in other browsers.

Bug: v8:5546
Cq-Include-Trybots: master.tryserver.v8:v8_linux_noi18n_rel_ng
Change-Id: I3e7c374c9b547f62d5976f76a7208d05fe9decf8
Reviewed-on: https://chromium-review.googlesource.com/581885
Commit-Queue: Kevin Gibbons <bakkot@gmail.com>
Reviewed-by: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47584}