clang-format: [JS] Support shebang lines on the very first line.

Summary:
Shebang lines (`#!/bin/blah`) can be used in JavaScript scripts to indicate
they should be run using e.g. node. This change treats # lines on the first line
as line comments.

Reviewers: djasper

Subscribers: klimek, cfe-commits

Differential Revision: http://reviews.llvm.org/D20632

llvm-svn: 271185