[Ada] Implement AI12-0028: Import of variadic C functions

[Ada] Implement AI12-0028: Import of variadic C functions

2020-06-09  Eric Botcazou  <ebotcazou@adacore.com>

gcc/ada/

	* exp_ch6.adb (Freeze_Subprogram): Deal with convention C_Family.
	* freeze.adb (Freeze_Profile): Likewise.  Add missing guard.
	* sem_mech.adb (Set_Mechanisms): Likewise.
	* lib-xref.adb (Output_Import_Export_Info): Ditto for C_Variadic.
	* repinfo.adb (List_Subprogram_Info): Likewise.
	* sem_prag.adb (Set_Convention_From_Pragma): Move main checks for
	Stdcall to...
	(Process_Convention): ...here.  Add checks for C_Variadic.
	* snames.ads-tmpl: Add Name_C_Variadic_0 .. Name_C_Variadic_16.
	Use consistent format for subtype declarations.
	(Convention_Id): Add Convention_C_Variadic_0 .. C_Variadic_16
	and move Convention_CPP up.
	(Convention_C_Family): New subtype of Convention_Id.
	(Convention_C_Variadic): Likewise.
	(Foreign_Convention): Use explicit upper bound.
	Add pragma Inline for Is_Configuration_Pragma_Name,
	Is_Function_Attribute_Name, Is_Internal_Attribute_Name
	and Is_Procedure_Attribute_Name.
	* snames.adb-tmpl (Get_Convention_Id): Deal with Name_Variadic_n.
	(Get_Convention_Name): Deal with Convention_Variadic_n.
	* types.h (Convention_Id): New typedef.
	* xsnamest.adb (Name2): New variable.
	(Is_Conv): New pattern.
	(Get_Subt1): Likewise.
	(Get_Subt2): Likewise.
	Output subtypes of Convention_Id into the C header file.