[Ada] Inheritance of Default_Iterator from interfaces

[Arnaud Charlet <charlet at adacore dot com>]

This patch fixes a bug in which if a type implements interfaces, the
Default_Iterator aspect is not inherited from all interfaces, but only
from the parent type.

The following test should compile and run quietly.

with Ada.Iterator_Interfaces;

procedure Main is

   type Element_Type is null record;

   type Cursor is null record;

   function Has_Element
     (Position : in Cursor)
      return Boolean
   is (False);

   package Iterators is new Ada.Iterator_Interfaces (Cursor, Has_Element);

   type Iterable_Base is limited interface
     with
       Default_Iterator => Iterate,
       Iterator_Element => Element_Type,
       Constant_Indexing => Element;

   type Iterator is new Iterators.Forward_Iterator with null record;

   function First
     (This : in Iterator)
      return Cursor
   is
     ((others => <>));

   function Next
     (This     : in Iterator;
      Position : in Cursor)
      return Cursor
   is
     (Position);

   function Iterate
     (This : in Iterable_Base'Class)
      return Iterators.Forward_Iterator'Class
   is
     (Iterator'(others => <>));

   function Element
     (This     : in Iterable_Base'Class;
      Position : in Cursor)
      return Element_Type
   is
     ((others => <>));

   type IDummy is limited interface;

   type Iterator_First is new Iterable_Base and IDummy with null record;

   type Iterator_Last is new IDummy and Iterable_Base with null record;

   A : Iterator_First;
   B : Iterator_Last;
begin

   for El of A loop
      null;
   end loop;

   for El of B loop
      null;
   end loop;
end Main;

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

2017-09-08  Bob Duff  <duff@adacore.com>

	* sem_ch3.adb (Build_Derived_Private_Type): Inherit
	representation items from interfaces that the derived type
	implements, not just from the parent type.
	* sem_util.ads, sem_util.adb (Abstract_Interface_List): Change
	this to return an empty list when there are no interfaces.
	* einfo.ads, sem_ch13.adb: Minor comment fixes.

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