[Ada] AI12-0382: Loosen type-invariant overriding requirement of AI12-0042

[Ada] AI12-0382: Loosen type-invariant overriding requirement of AI12-0042

gcc/ada/

	* sem_ch3.adb (Check_Abstract_Overriding): Remove Scope
	comparison test from test related to initial implementation of
	AI12-0042, plus remove the related ??? comment.
	(Derive_Subprogram): Add test requiring that the type extension
	appear in the visible part of its enclosing package when
	checking the overriding requirement of 7.3.2(6.1/4), as
	clarified by AI12-0382.