[Ada] Avoid copy operation for returns involving function calls

gcc/ada/

	* gcc-interface/decl.cc (gnat_to_gnu_entity) <E_Constant>: Deal with
	a constant related to a return in a function specially.
	* gcc-interface/trans.cc (Call_to_gnu): Use return slot optimization
	if the target is a return object.
	(gnat_to_gnu) <N_Object_Declaration>: Deal with a constant related
	to a return in a function specially.