ada: Fix crash on iterator of type with Constant_Indexing aspect

ada: Fix crash on iterator of type with Constant_Indexing aspect

This happens when the type returned by the indexing function is a private
type whose completion is derived from another private type, because the
Finalize_Address routine cannot correctly fetch the actual root type.

gcc/ada/ChangeLog:

	* exp_util.adb (Finalize_Address): In an untagged derivation, call
	Root_Type on the full view of the base type if the partial view is
	itself not a derived type.
	(Is_Untagged_Derivation): Minor formatting tweak.