intl_date_time_format_function is only set in i18n builds. Use
opaque_reference_function to test for Builtins::kIllegal instead.

TBR=yangguo@chromium.org
NOTREECHECKS=true
NOTRY=true

Bug: v8:6624
Change-Id: I0631862e6e022d222b52dceb58a8fe65f7d6a4e8
Reviewed-on: https://chromium-review.googlesource.com/643506Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47714}