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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@169605 91177308-0d34-0410-b5e6-96231b3b80d8