ada: Reject illegal array aggregates as per AI22-0106.

Implement the new legality rules of AI22-0106 which (as discussed in the AI)
are needed to disallow constructs whose semantics would otherwise be poorly
defined.

gcc/ada/

	* sem_aggr.adb (Resolve_Array_Aggregate): Implement the two new
	legality rules of AI11-0106. Add code to avoid cascading error
	messages.