Split long lines in multi-line comments.

Summary: This is implementation for /* */ comments only.

Reviewers: djasper, klimek

Reviewed By: djasper

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D547

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@177415 91177308-0d34-0410-b5e6-96231b3b80d8