Add support for 1->N type mappings in the dialect conversion infrastructure....

Add support for 1->N type mappings in the dialect conversion infrastructure. To support these mappings a hook must be overridden on the type converter: 'materializeConversion' :to generate a cast operation from the new types to the old type. This operation is automatically erased if all uses are removed, otherwise it remains in the IR for the user to handle.

PiperOrigin-RevId: 254411383