Don't break line comments with escaped newlines.

Summary:
These can appear when comments contain command lines with quoted line
breaks. As the text (including escaped newlines and '//' from consecutive lines)
is a single line comment, we used to break it even when it didn't exceed column
limit. This is a temporary solution, in the future we may want to support this
case completely - at least adjust leading whitespace when changing indentation
of the first line.

Reviewers: djasper

Reviewed By: djasper

CC: cfe-commits, klimek

Differential Revision: http://llvm-reviews.chandlerc.com/D1146