Clang-format: extracted FormatTokenSource from UnwrappedLineParser.

Summary: FormatTokenLexer is here, FormatTokenBuffer is on the way. This will allow to re-parse unwrapped lines when needed.

Reviewers: djasper, klimek

Reviewed By: klimek

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D186

llvm-svn: 169605