[Ada] update based on AI-216

[Arnaud Charlet <charlet at adacore dot com>]

Tested under i686-linux, commited on mainline.

AI-216 defines various conditions in which use of an unchecked union type
must raise program_error. Among those are some cases of equality, and
stream operations. When an unchecked union is a component of a tagged type,
calls to any of these predefined operations will raise program_error.
GNAT incorrectly inserted the raise statement before the body of the
equality operation, rather than within it.

The following example must execute quietly:
--
procedure p is
   type Unc (Disc : Boolean := False) is record
     case Disc is
        when True =>  V1 : Long_Integer;
        when False => V2 : Float;
     end case;
   end record;
   pragma Unchecked_Union (Unc);
   type Wrapper is tagged record Thing : Unc; end record;
   Obj : Wrapper;
begin
   null;
end;

2005-02-09  Ed Schonberg  <schonberg@adacore.com>

	* exp_ch4.adb (Expand_Composite_Equality): If a component is an
	unchecked union with no inferable discriminants, return a
	Raise_Program_Error node, rather than inserting it at the point the
	type is frozen.
	(Expand_Record_Equality, Component_Equality): Handle properly the case
	where some subcomponent is an unchecked union whose generated equality
	code raises program error.

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