Added Lexer::getBufferEnd().

Summary:
There's Lexer::getBufferStart(), and we need getBufferEnd() to access
the whole input buffer in clang::format::reformat. We don't want to
rely on the fact that the Lexer::BufferEnd always points to '\0', as there can
be embedded '\0's as well.

Reviewers: jordan_rose

Reviewed By: jordan_rose

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D916