[Ada] Spurious access error in function returning type with access discriminant

[Ada] Spurious access error in function returning type with access discriminant

This patch fixes an issue in the compiler whereby incorrect
accessibility checks were generated in functions returning types with
unconstrained access discriminants when the value supplied for the
discriminant is a formal parameter.

More specifically, accessibility checks for return statements featuring
a result type having access discriminants were incorrectly being
performed against the level of the function declaration instead of the
level of the master of the call.

gcc/ada/

	* sem_ch6.adb (Check_Return_Construct_Accessibility): Modify
	generation of run-time accessibility checks to account for cases
	where Extra_Accessibility_Of_Result should be used versus the
	level of the enclosing subprogram. Use original node to avoid
	checking against expanded code. Disable check generation for
	tagged type case.
	(Is_Formal_Of_Current_Function): Added to encompass a predicate
	used within Check_Return_Construct_Accessibility to test if an
	associated expression is related to a relevant formal.
	* sem_util.adb, sem_util.ads (Enclosing_Subprogram): Modified to
	accept Node_Or_Entity_Id.
	(Innermost_Master_Scope_Depth): Calculate level based on the
	subprogram of a return statement instead of the one returned by
	Current_Subprogram.
	(Needs_Result_Accessibility_Level): Remove
	Disable_Coextension_Cases constant, and disable the tagged type
	case for performance reasons.