[Ada] Take full view of private type

[Ada] Take full view of private type

This allows to resolve the following:

      type Rec (<>) is private;
      type Arr (<>) is private;
   private
      type Arr is array (Positive range <>) of Natural;
      type Rec (L : Natural) is record
         F1 : Integer;
         F2 : Arr (1 .. L);
      end record;

gcc/ada/

	* sem_ch3.adb (Analyze_Subtype_Declaration): Use underlying type
	of Indic_Typ.
	(Constrain_Array): Ditto for T.