Do not call Set_Cloned_Subtype on private type

Build_Discriminated_Subtype may be invoked on a E_Record_Type_With_Private,
in which case it builds a E_Record_Subtype_With_Private which does not have
the Cloned_Subtype field.

gcc/ada/
	PR ada/99020
	* sem_ch3.adb (Build_Discriminated_Subtype): Set the Cloned_Subtype
	only if the type is not private.

gcc/ada/sem_ch3.adb

@@ -10493,7 +10493,7 @@ package body Sem_Ch3 is
          then
             Create_Constrained_Components (Def_Id, Related_Nod, T, Elist);
 
-         else
+         elsif not Is_Private_Type (T) then
             Set_Cloned_Subtype (Def_Id, T);
          end if;
       end if;