Added LanguageStandard::LS_JavaScript to gate all JS-specific parsing.

Summary:
Use LS_JavaScript for files ending with ".js". Added support for ">>>="
operator.

Reviewers: djasper, klimek

Reviewed By: djasper

CC: cfe-commits, klimek

Differential Revision: http://llvm-reviews.chandlerc.com/D2242

llvm-svn: 195961