ada: Small housekeeping work in Exp_Aggr

This moves a few declarations around and tweaks a few comments.

gcc/ada/ChangeLog:

	* exp_aggr.adb (Case_Table_Type): Fix reference in comment.
	(In_Place_Assign_OK): Move declaration around.
	(Is_Build_In_Place_Aggregate_Return): Likewise and adjust.
	(Expand_Array_Aggregate): Streamline for the sake of consistency.
	(Aggr_Assignment_OK_For_Backend): Remove reference to Gigi/gcc.
	(Backend_Processing_Possible): Likewise.
	(Expand_Array_Aggregate): Add comment.