Currently, we don't check if the parentID of a multi-region type descriptor is valid. This is because the multi region type descriptor may be created as part of the same transaction as the database descriptor, which means we can't read the db desc from kv during validation. This could be fixed by making the validation first go through the desc collection before reading descriptors from kv. Once we've got this sorted, we can also add an array type alias for these type descriptors -- currently the only thing stopping us from creating an array alias for multi-region enum kinds is validation failure.