[Ada] Spurious error with component of unchecked_union type

gcc/ada/

	* exp_ch4.adb (Unconstrained_UU_In_Component_Declaration): A
	component declaration whose subtype indication is an entity name
	without an explicit constraint is an Unchecked_Union type only
	if the entity has an unconstrained nominal subtype (record type
	or private type) whose parent type is an Unchecked_Union.