Invalid use of literals in expressions

[treiher]

The invalid use of literals in expressions leads to a bug box.

package Test is

   type T is (X, Y) with Size =>  8;

   type M is
      message
         A : T
            then null
               if X = Y + 1;  --  Invalid
      end message;

end Test;

This issue is related to #686. Both issues can be fixed by detecting invalidly used literals.