[Ada] Spurious error on private task derivation

The compiler reports a spurious error notifying a missing constraint in the
declaration of a private type with discriminants whose full view is a
derivation of a task type.

After this patch the following test compiles without errors.

package Types1 is
   type Parent (Discr1 : Boolean) is limited private;
private
   task type Parent (Discr1 : Boolean);
end Types1;

with Types1; use Types1;
package Types2 is
   type Child (Discr2 : Boolean) is limited private;
private
   type Child (Discr2 : Boolean) is       -- Test
     new Parent (Discr1 => Discr2);
end Types2;

Command: gcc -c types2.ads

2018-05-24  Javier Miranda  <miranda@adacore.com>

gcc/ada/

	* sem_util.adb (Abstract_Interface_List): Add missing support for
	private types whose full view is a synchronized type.
	* sem_ch3.adb (Build_Derived_Private_Type): Skip building the full
	derivation of a private type parent type is a task type with
	discriminants as gigi does not use such type directly.

From-SVN: r260644

gcc/ada/ChangeLog

+2018-05-24  Javier Miranda  <miranda@adacore.com>
+
+	* sem_util.adb (Abstract_Interface_List): Add missing support for
+	private types whose full view is a synchronized type.
+	* sem_ch3.adb (Build_Derived_Private_Type): Skip building the full
+	derivation of a private type parent type is a task type with
+	discriminants as gigi does not use such type directly.
+
 2018-05-24  Hristian Kirtchev  <kirtchev@adacore.com>
 
 	* sem_elab.adb (Build_Variable_Reference_Marker): Do not create a

gcc/ada/sem_ch3.adb

@@ -7856,12 +7856,12 @@ package body Sem_Ch3 is
          --  Build the full derivation if this is not the anonymous derived
          --  base type created by Build_Derived_Record_Type in the constrained
          --  case (see point 5. of its head comment) since we build it for the
-         --  derived subtype. And skip it for protected types altogether, as
+         --  derived subtype. And skip it for synchronized types altogether, as
          --  gigi does not use these types directly.
 
          if Present (Full_View (Parent_Type))
            and then not Is_Itype (Derived_Type)
-           and then not (Ekind (Full_View (Parent_Type)) in Protected_Kind)
+           and then not (Is_Concurrent_Type (Full_View (Parent_Type)))
          then
             declare
                Der_Base   : constant Entity_Id := Base_Type (Derived_Type);

gcc/ada/sem_util.adb

@@ -184,11 +184,11 @@ package body Sem_Util is
          --  If we are dealing with a synchronized subtype, go to the base
          --  type, whose declaration has the interface list.
 
-         --  Shouldn't this be Declaration_Node???
+         Nod := Declaration_Node (Base_Type (Typ));
 
-         Nod := Parent (Base_Type (Typ));
-
-         if Nkind (Nod) = N_Full_Type_Declaration then
+         if Nkind_In (Nod, N_Full_Type_Declaration,
+                           N_Private_Type_Declaration)
+         then
             return Empty_List;
          end if;