This patch adds support for Regional_Indicator within Unicode property
escapes in regular expressions.

The Regional_Indicator binary property was added in Unicode v10 and is
supported in ICU 60.1.

An `#if` directive is used to prevent breaking Node.js until they
update their ICU.

BUG=v8:4743

Cq-Include-Trybots: master.tryserver.v8:v8_linux_noi18n_rel_ng
Change-Id: I7acec13c8ae7552558a0f852937984bba828e738
Reviewed-on: https://chromium-review.googlesource.com/758273Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Mathias Bynens <mathias@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49250}