[Ada] Another fix in handling of interfaces

[Arnaud Charlet <charlet at adacore dot com>]

Tested on i686-linux, committed on HEAD

Gather_Component is called to collect the full list of components of a
derived record, in order to analyze an aggregate of the type. The list
must include only explicit components that must appear in the aggregate,
and omit the tag and internal subtypes. In Ada2005, it must also exclude
the interface tags that are present when the type of the object implements
an interface.
The following must compile quietly:

package Aggr_Bug is
   type I is Interface;
   type O is tagged null record;

   type P is new O and I with null record;
   Value1 : P := (O with null record);

   type Q is new O and I with record
      Num : Integer;
   end record;

   Value2 : Q := (O with 15);
   Value3 : Q := (O with Num => 15);
   Value4 : Q := (Num => 15);
end Aggr_Bug;

The front-end performs some constant propagation in straight-line code,
and Safe_To_Capture_Value determines whether an inferred value can safely be
reused in subsequent expressions. In the absence of more complex data-
flow analysis, the front-end does not store value information that is
generated in code that might not be executed, such as a given branch
of an if-statement. The case of the second operand of a short-circuit
expression was simply missing.
The following must compile and execute silently:
--
procedure short is
   type rec is record value : integer := 10; end record;
   type T is access Rec;
   Ptr : T := new Rec;
   Maybe : boolean;
begin
   if ptr.all.value = 10 then ptr := null; end if;
   maybe := ptr /= null and then ptr.all.value = 10;
end;

2005-09-01  Ed Schonberg  <schonberg@adacore.com>

	* sem_util.ads, sem_util.adb (Gather_Components): Omit interface tags
	from the list of required components.
	(Is_Controlling_Limited_Procedure): Determine whether an entity is a
	primitive procedure of a limited interface with a controlling first
	parameter.
	(Is_Renamed_Entry): Determine whether an entry is a procedure renaming
	of an entry.
	(Safe_To_Capture_Value): A value (such as non_null) is not safe to
	capture if it is generated in the second operand of a short-circuit
	operation.
	Do not capture values for variables with address clauses.
	(Is_Object_Reference): Treat a function call as an object reference only
	if its type is not Standard_Void_Type.

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