[Ada] Cleanup insertion of single freezing actions

[Ada] Cleanup insertion of single freezing actions

gcc/ada/

	* exp_util.adb (Append_Freeze_Action): Tune whitespace to make
	the code look similar to Append_Freeze_Actions, which takes a
	List_Id.
	* sem_ch6.adb (Analyze_Return_Type): Cleanup with
	Append_Freeze_Action.
	* exp_ch3.adb (Build_Access_Subprogram_Wrapper_Body): Likewise.
	* sem_ch3.adb (Build_Access_Subprogram_Wrapper): Likewise.
	* contracts.adb (Add_Indirect_Call_Wrapper): Remove extra call
	to Ensure_Freeze_Node.
	(Add_Call_Helper): Likewise.
	* freeze.adb (Check_Inherited_Conditions): Likewise.
	(Attribute_Renaming): Likewise.
	* sem_ch8.adb: Likewise.