clang-format: Improve clang-format's detection about comment binding.

Before, existing code in the form of:

  int a; // this is a.
  // This is
  // b.
  int b;

Got turned into:

  int a; // this is a.
         // This is
  // b.
  int b;

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@194294 91177308-0d34-0410-b5e6-96231b3b80d8