[Ada] Spurious error on private task derivation

[Pierre-Marie de Rodat]

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

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

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.
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