ada: Fix internal error on invalid prefix with assertions enabled

ada: Fix internal error on invalid prefix with assertions enabled

This happens for example with:

package Q3 is
  type Types is (One, Two);
end Q3;

with Q3;

package P3 is
  Kind : Q3.Types := Q3.Types.One;
end P3;

and prevents the error from being given.

gcc/ada/ChangeLog:

	PR ada/112979
	* sem_ch8.adb (Find_Selected_Component): Try to recognize the
	object operation notation only if the selector is a subprogram.