[Ada] Robust detection of access-to-subprogram and access-to-object types

gcc/ada/

	* einfo-utils.adb (Is_Access_Object_Type): Use
	Directly_Designated_Type.
	(Is_Access_Subprogram_Type): Use Directly_Designated_Type.
	(Set_Convention): Use plain Ekind.
	* gen_il-gen-gen_entities.adb (Type_Kind): Use plain Ekind.
	* sem_ch3.adb (Access_Type_Declaration): When seeing an illegal
	completion with an access type don't attempt to decorate the
	completion entity; previously the entity had its Ekind set to
	E_General_Access_Type or E_Access_Type, but its Designated_Type
	was empty, which caused a crash in freezing. (Actually, the
	error recovery in the surrounding context is still incomplete,
	e.g. we will crash when the illegal completion is an access to
	an unknown identifier).