ada: Fix invalid JSON for extended variant record with -gnatRj

ada: Fix invalid JSON for extended variant record with -gnatRj

This fixes the output of -gnatRj for an extension of a tagged type which has
a variant part and also deals with the case where the parent type is private
with unknown discriminants.

gcc/ada/

	* repinfo.ads (JSON output format): Document special case of
	Present member of a Variant object.
	* repinfo.adb (List_Structural_Record_Layout): Change the type of
	Ext_Level parameter to Integer. Restrict the first recursion with
	increasing levels to the fixed part and implement a second
	recursion with decreasing levels for the variant part. Deal with
	an extension of a type with unknown discriminants.