[Ada] Legal actual type with inherited discriminants rejected in instantiation

[Ada] Legal actual type with inherited discriminants rejected in instantiation

gcc/ada/

	* sem_eval.adb (Subtypes_Statically_Match): Retrieve
	discriminant constraints from the two types via new function
	Original_Discriminant_Constraint rather than
	Discriminant_Constraint.
	(Original_Discriminant_Constraint): New function to locate the
	nearest explicit discriminant constraint associated with a type
	that may possibly have inherited a constraint from an ancestor
	type.