Typo in `is`

[purpleKarrot]

I am sure the code in

https://github.com/hsutter/cppfront/blob/fd0c99d253da3f11dab3027af88cd32b4f27f608/include/cpp2util.h#L539-L543

should cast to a pointer rather than to a reference:

  return dynamic_cast<C const*>(x) != nullptr;