Fix comment alignment behavior.

In the following snippet, clang-format incorrectly aligned the
trailing comment, when only the last line was formatted:

  int aaaaaa; // comment
  int b;
  int c; // Formatting only this line moved this comment.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@180173 91177308-0d34-0410-b5e6-96231b3b80d8