[Ada] Fix invalid JSON for derived variant record with -gnatRj

[Ada] Fix invalid JSON for derived variant record with -gnatRj

gcc/ada/

	* repinfo.ads (JSON output format): Document adjusted key name.
	* repinfo.adb (List_Record_Layout): Use Original_Record_Component
	if the normalized position of the component is not known.
	(List_Structural_Record_Layout): Rename Outer_Ent parameter into
	Ext_End and add Ext_Level parameter. In an extension, if the parent
	subtype has static discriminants, call List_Record_Layout on it.
	Output "parent_" prefixes before "variant" according to Ext_Level.
	Adjust recursive calls throughout the procedure.