ada: Allow making empty aggregates positional

ada: Allow making empty aggregates positional

This patch makes Exp_Aggr.Convert_To_Positional accepts appropriate
empty aggregates. The end goal is to remove violations of the
No_Elaboration_Code restriction in some cases of library-level array
objects.

gcc/ada/

	* exp_aggr.adb (Flatten): Do not reject empty aggregates. Adjust
	criterion for emitting warning about ineffective others clause.
	* sem_aggr.adb (Array_Aggr_Subtype): Fix typo. Add handling of
	aggregates that were converted to positional form.
	(Resolve_Aggregate): Tweak criterion for transforming into a
	string literal.
	(Resolve_Array_Aggregate): Tweak criterion for reusing existing
	bounds of aggregate.
	(Retrieve_Aggregate_Bounds): New procedure.
	* sem_util.adb (Has_Static_Empty_Array_Bounds): New function.
	* sem_util.ads (Has_Static_Empty_Array_Bounds): Likewise.