Preserve hanging indent when breaking line comments.

Summary:
If we need to break the second line here:
// something: aaaaa aaaaa aaaaaa aaaaa aaaaa
//            aaaaa aaaaa aaaaaa aaaaa aaaaa aaaaa

with the patch it will be turned to

// something: aaaaa aaaaa aaaaaa aaaaa aaaaa
//            aaaaa aaaaa aaaaaa aaaaa aaaaa
//            aaaaa

instead of

// something: aaaaa aaaaa aaaaaa aaaaa aaaaa
//            aaaaa aaaaa aaaaaa aaaaa aaaaa
// aaaaa

Reviewers: djasper, klimek

Reviewed By: klimek

CC: cfe-commits, klimek

Differential Revision: http://llvm-reviews.chandlerc.com/D2988

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@203458 91177308-0d34-0410-b5e6-96231b3b80d8