clang-format: Don't merge lines with comments.

Before:
  int f() { // comment return 42; }

After:
  int f() { // comment
    return 42;
  }

This fixes llvm.org/PR21769.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@223609 91177308-0d34-0410-b5e6-96231b3b80d8