clang-format: Improve heuristic around avoiding bad line breaks.

Now, this can be properly formatted:
  static_cast<A< //
      B> *>(     //
      );

Before, clang-format could end up, not formatting the code at all.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@213055 91177308-0d34-0410-b5e6-96231b3b80d8