BVAL-361, BVAL-358

* [BVAL-361](https://hibernate.onjira.com/browse/BVAL-361) Adding getGroupConversions() for cascadable descriptor types
* [BVAL-358](https://hibernate.onjira.com/browse/BVAL-361) Make ExecutableDescriptor#validateConstructorParameters() and validateConstructorReturnValue() more usable