[Ada] Create constrained itypes for nested record aggregates

[Ada] Create constrained itypes for nested record aggregates

2020-06-11  Piotr Trojanek  <trojanek@adacore.com>

gcc/ada/

	* sem_aggr.adb (Build_Constrained_Itype): Previously a declare
	block, now a separate procedure; the only change is that now
	New_Assoc_List might include components and an others clause,
	which we ignore (while we deal with discriminants exactly as we
	did before); extend a ??? comment about how this routine is
	different from the Build_Subtype
	(Resolve_Record_Aggregate): Create a constrained itype not just
	for the outermost record aggregate, but for its inner record
	aggregates as well.