Better block comment formatting.

Summary:
1. When splitting one-line block comment, use indentation and *s.
2. Remove trailing whitespace from all lines of a comment, not only the ones being splitted.
3. Add backslashes for all lines if a comment is used insed a preprocessor directive.

Reviewers: djasper

Reviewed By: djasper

CC: cfe-commits, klimek

Differential Revision: http://llvm-reviews.chandlerc.com/D557

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@177635 91177308-0d34-0410-b5e6-96231b3b80d8