Fix the CASTing behavior of UDTs

The actual behavior of UDTs, now under construction, does not conform to the spec. The spec says that you can't CAST to or from UDTs. This is probably too harsh. We should allow the following CAST

cast ( null as udtType )

I also think that we should allow at least the implicit casting of subtypes to supertypes since Java gives us the ability to discover the type graph. See Knut's comment on DERBY-651 on Dec 11, 2009.