ada: Fix assertion failure on illegal use of aspect Type_Invariant

ada: Fix assertion failure on illegal use of aspect Type_Invariant

The illegal use is on a type derived from a formal private type, e.g.:

generic

  type T is private;

package G is

  type D is new T with Type_Invariant => True;

end G;

gcc/ada/ChangeLog:

	PR ada/113037
	* sem_prag.adb (Analyze_Pragma) <Pragma_Invariant>: Reject types
	that are derived from formal private types.