[Ada] Implementation of AI05-213 : formal incomplete types

[Arnaud Charlet <charlet at adacore dot com>]

Ada2012 introduce a new kind of formal type definition. An incomplete formal
type can be instantiated with any actual (as long as discriminants and tagged 
nature conform).  The actual for a formal incomplete type is not frozen by
the instance itself.

The following must compile quietly in Ada2012 mode:

---
procedure test1 is
   generic
      type Later;
   package G is
      X : Integer;
   end G;

   package Inst is new G (Integer);
    generic
       type Latest is tagged;
    package G2 is
       It : Float;
    end;
 
    type TT is tagged null record;
    package Inst2 is new G2 (TT);

    package Inner is
       type T;
       package Inst3 is new G (T);
       type T is array (1..10) of integer;
    private
   end Inner;

    package Inner2 is
       type T is private;
       package Inst3 is new G (T);
    private
       type T is array (1..10) of integer;
   end Inner2;
begin
  null;
end;

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

2011-08-29  Ed Schonberg  <schonberg@adacore.com>

	* sinfo.ads, sinfo.adb: New node kind
	N_Formal_Incomplete_Type_Definition, related flags.
	par-ch12.adb (P_Formal_Type_Declaration, G_Formal_Type_Definition):
	Parse formal incomplete types.
	* sem.adb (Analyze): Formal_Incomplete_Type_Definitions are handled in
	sem_ch12.
	* sem_ch7.adb (Analyze_Package_Specification, Unit_Requires_Body):
	Formal incomplete types do not need completion.
	* sem_ch12.adb (Analyze_Formal_Incomplete_Type,
	Validate_Incomplete_Type_Instance): New procedures to handle formal
	incomplete types.
	* freeze.adb (Freeze_Entity): Do not freeze the subtype of an actual
	that corresponds to a formal incomplete type.
	* sprint.adb: Handle formal incomplete type declarations.
	* exp_util.adb (Insert_Actions): An incomplete_type_definition is not
	an insertion point.

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