clang-format: [JS] exclaim preceding regex literals.

Summary:
Regex detection would incorrectly classify a trailing `!` operator
(nullability cast) followed by a `/` as the start of a regular
expression literal. This fixes code such as:

    var foo = x()! / 10;

Which would previously parse a regexp all the way to the end of the
source file (or next `/`).

Reviewers: djasper

Subscribers: cfe-commits, klimek

Differential Revision: https://reviews.llvm.org/D29634

llvm-svn: 294304