Align comments to surrounding unformatted comments.

Before:
int a; // not formatted
// formatting this line only

After:
int a; // not formatted
       // formatting this line only

This makes clang-format stable independent of whether the whole
file or single lines are formatted in most cases.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@177739 91177308-0d34-0410-b5e6-96231b3b80d8