IdentifierStart::Is and IdentifierContinue::Is both return true for '\'.
The reason for this is lost to history.

Special-case '\' in the regexp parser to handle this.

BUG=v8:5437,v8:5868

Review-Url: https://codereview.chromium.org/2795093003
Cr-Commit-Position: refs/heads/master@{#44396}