Let the formatter align trailing line comments where possible.

Before:
int a; // comment
int bbbbb; // comment

After:
int a;     // comment
int bbbbb; // comment

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@172798 91177308-0d34-0410-b5e6-96231b3b80d8