Add predicates on subtypes E and N.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

	* einfo-utils.ads, einfo-utils.adb: Add predicates on subtypes E
	and N.  Change some parameters to use the unpredicated subtypes,
	because they sometimes return e.g. Empty.  Note that N_Entity_Id
	has a predicate; Entity_Id does not.
	* exp_tss.adb (Base_Init_Proc): Use Entity_Id instead of E,
	because otherwise we fail the predicate. We shouldn't be
	referring to single-letter names from far away anyway.
	* sem_aux.adb (Is_Derived_Type): Likewise.
	* sem_res.adb (Is_Definite_Access_Type): Use N_Entity_Id for
	predicate.
	* types.ads (Entity_Id): Add comment explaining the difference
	between Entity_Id and N_Entity_Id.