[Ada] Ada2020: AI12-0107 convention of By_Protected_Procedure

[Ada] Ada2020: AI12-0107 convention of By_Protected_Procedure

gcc/ada/

	* exp_attr.adb (Has_By_Protected_Procedure_Prefixed_View): New
	subprogram.
	(Expand_Access_To_Protected_Op): Adding support for prefixed
	class-wide view with By_Protected_Procedure convention.
	* sem_attr.adb (Get_Convention): New subprogram.
	(Get_Kind): Adapted to use Get_Convention.
	* sem_ch4.adb (Try_By_Protected_Procedure_Prefixed_View): New
	subprogram.
	(Analyze_Selected_Component): Invoke
	Try_By_Protected_Procedure_Prefixed_View.
	* sem_util.ads (Is_By_Protected_Procedure): New subprogram.
	* sem_util.adb (Is_By_Protected_Procedure): New subprogram.