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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@189595 91177308-0d34-0410-b5e6-96231b3b80d8