[Ada] Warn on unset objects in packages with no bodies

[Ada] Warn on unset objects in packages with no bodies

Fix an inconsistency, where GNAT was warning about references to unset
objects inside generic packages with no bodies but not inside ordinary
packages with no bodies.

gcc/ada/

	* sem_ch7.adb (Analyze_Package_Declaration): Check references to
	unset objects.

gcc/testsuite/

	* gnat.dg/specs/discr5.ads: Expect new warnings.
	* gnat.dg/specs/empty_variants.ads: Likewise.
	* gnat.dg/specs/pack13.ads: Likewise.