[Ada] Fix inaccurate -gnatR output for derived untagged types

[Ada] Fix inaccurate -gnatR output for derived untagged types

gcc/ada/

	* repinfo.adb (Compute_Max_Length): Skip hidden discriminants.
	(List_Record_Layout): Likewise.
	(List_Structural_Record_Layout): Use First_Discriminant instead
	of First_Stored_Discriminant and Next_Discriminant instead of
	Next_Stored_Discriminant to walk the list of discriminants.