[Ada] Missing errors on extension aggregates with unknown discriminants

[Ada] Missing errors on extension aggregates with unknown discriminants

gcc/ada/

	* sem_aggr.adb (Resolve_Extension_Aggregate): When testing for
	an aggregate that is illegal due to having an ancestor type that
	has unknown discriminants, add an "or else" condition testing
	whether the aggregate type has unknown discriminants and that
	Partial_View_Has_Unknown_Discr is also set on the ancestor type.
	Extend the comment, including adding ??? about a possible
	simpler test.