ada: Cleanup in expansion of array aggregates in object declarations

This mainly decouples the handling of the declaration case from that of the
assignment case in Expand_Array_Aggregate, as well as moves the expansion
in the case of an aggregate that can be processed by the back end to the
Build_Array_Aggr_Code routine.

gcc/ada/ChangeLog:

	* exp_aggr.adb (Build_Array_Aggr_Code): Build the simple assignment
	for the case of an aggregate that can be handled by the back end.
	(Expand_Array_Aggregate): Adjust description of the processing.
	Move handling of declaration case to STEP 4 and remove handling of
	the case of an aggregate that can be processed by the back end.
	(Late_Expansion): Likewise for the second part.
	* exp_ch3.adb (Expand_N_Object_Declaration): Deal with a delayed
	aggregate synthesized for the default initialization, if any.
	* sem_eval.adb (Eval_Indexed_Component): Bail out for the name of
	an assignment statement.