clang-format: [JS] sort ES6 imports.

Summary:
This change automatically sorts ES6 imports and exports into four groups:
absolute imports, parent imports, relative imports, and then exports. Exports
are sorted in the same order, but not grouped further.

To keep JS import sorting out of Format.cpp, this required extracting the
TokenAnalyzer infrastructure to separate header and implementation files.

Reviewers: djasper

Subscribers: cfe-commits, klimek

Differential Revision: http://reviews.llvm.org/D20198

llvm-svn: 270203