Indent all lines in a multi-line comment by the same amount.

Summary:
Do this to avoid spoling nicely formatted multi-line comments (e.g.
with code examples or similar stuff).

Reviewers: djasper

Reviewed By: djasper

CC: cfe-commits, klimek

Differential Revision: http://llvm-reviews.chandlerc.com/D544

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@177153 91177308-0d34-0410-b5e6-96231b3b80d8