ada: Crash on creation of extra formals on type extension

ada: Crash on creation of extra formals on type extension

The compiler blows up processing an overriding dispatching function
of a derived tagged type that returns a private tagged type that
has an access type discriminant.

gcc/ada/

	* accessibility.ads (Needs_Result_Accessibility_Extra_Formal): New
	subprogram.
	* accessibility.adb (Needs_Result_Accessibility_Level_Param): New
	subprogram.
	(Needs_Result_Accessibility_Extra_Formal): New subprogram,
	temporarily keep the previous behavior of the frontend.
	* sem_ch6.adb (Create_Extra_Formals): Replace occurrences of
	function Needs_Result_Accessibility_Level_Param by calls to
	function Needs_Result_Accessibility_Extra_Formal.
	(Extra_Formals_OK): Ditto.