[Ada] Crash on function in Ghost subunit

[Ada] Crash on function in Ghost subunit

This patch modifies the creation of class-wide subtypes to preserve vital
attributes related to Ghost code. The subtype is created by copying the
contents of a class-wide type into a newly created itype. When the itype
is created within a Ghost region, the act of copying destroys Ghost code
related attributes. As a result, if the now living class-wide subtype is
frozen within an ignored Ghost region, its freeze node is hoisted prior
to the start of the region, howeve the subtype is still eliminated from
the tree.

------------
-- Source --
------------

--  pack.ads

with Ada.Finalization; use Ada.Finalization;

package Pack is
   type Ctrl is new Controlled with null record;
   function Make_Ctrl return Ctrl;

   package Nested with Ghost is
      procedure Proc;
   end Nested;
end Pack;

--  pack.adb

package body Pack is
   function Make_Ctrl return Ctrl is
   begin
      return Result : Ctrl;
   end Make_Ctrl;

   package body Nested is separate;
end Pack;

--  pack-nested.adb

separate (Pack)

package body Nested is
   procedure Proc is
      Res : constant Ctrl'Class := Make_Ctrl;
   begin null; end Proc;
end Nested;

-----------------
-- Compilation --
-----------------

$ gcc -c pack.adb

2018-05-24  Hristian Kirtchev  <kirtchev@adacore.com>

gcc/ada/

	* exp_util.adb (New_Class_Wide_Subtype): Capture and restore relevant
	Ghost-related attributes of the class-wide subtype because the copy
	clobbers them.

From-SVN: r260653