ada: Fix small inaccuracy in implementation of B.3.3(20/2)

ada: Fix small inaccuracy in implementation of B.3.3(20/2)

This is the clause about inferable discriminants in unchecked unions.

gcc/ada/

	* sem_util.adb (Has_Inferable_Discriminants): In the case of a
	component with a per-object constraint, also return true if the
	enclosing object is not of an unchecked union type.
	In the default case, remove a useless call to Base_Type.