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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@195961 91177308-0d34-0410-b5e6-96231b3b80d8