Better support for multiline string literals (including C++11 raw string literals).

Summary:
Calculate characters in the first and the last line correctly so that
we only break before the literal when needed.

Reviewers: djasper

Reviewed By: djasper

CC: cfe-commits, klimek

Differential Revision: http://llvm-reviews.chandlerc.com/D1544

llvm-svn: 189595