Don't remove backslashes from block comments.

Summary:
Don't remove backslashes from block comments. Previously this
/* \    \ \ \ \ \
*/
would be turned to this:
/*
*/
which spoils some kinds of ASCII-art, people use in their comments. The behavior
was related to handling escaped newlines in block comments inside preprocessor
directives. This patch makes handling it in a more civilized way.

Reviewers: klimek

Reviewed By: klimek

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D979