clang-format: Improve selective comment formatting.

Starting here:
  int x; // Format this line only.
  int xx; //
  int xxxxx; //

Before:
  int x;     // Format this line only.
  int xx; //
  int xxxxx; //

After:
  int x;  // Format this line only.
  int xx; //
  int xxxxx; //

llvm-svn: 257258