Detecting invalid message with two field sizes defined by message size

[treiher]

A message specification where two field sizes are defined by the message size should not be accepted as valid.

package Test is

   type M is
      message
         A : Opaque
            with Size => Message'Size;
         B : Opaque
            with Size => Message'Size - A'Size;
      end message;

end Test;

[treiher]

This will be prevented by #555.

[treiher]

The issue will be obsolete after realization of #736, as the use of Message'Size in size aspects will be prohibited.