[Ada] Improve allocation of constants with discriminated types

[Eric Botcazou]

As demonstrated by the following testcase, the allocation of constants with 
discriminated types in Gigi is suboptimal.  For

   type T (Big : Boolean := False) is record
      case Big is
         when True =>
            Content : Integer;
         when False =>
            null;
       end case;
    end record;

    D : constant T := (True, 0);

    Var :          T := D;
    Con : constant T := D;

D is allocated statically with exact size, Var is allocated statically with 
maximum size, but Con is allocated dynamically (with exact size).

We can do better by propagating the size of D thanks to its constantness.

Tested on i586-suse-linux, applied on the mainline


2008-04-02  Eric Botcazou  <ebotcazou@adacore.com>

	* decl.c (gnat_to_gnu_entity) <object>: For a constant object whose
	type has self-referential size, get the size from the initializing
	expression directly if it is also a constant whose nominal type
	has self-referential size.


2008-04-02  Eric Botcazou  <ebotcazou@adacore.com>

	* gnat.dg/specs/discr_record_constant.ads: New test.


-- 
Eric Botcazou
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