Better support for ranges

[NlightNFotis]

During the work done in #171 to support the case statement in ADA programs, we found out that ranges are not very well supported. As a result a test that contains ranges in case statement conditions has been added but marked XFAIL. It looks like this:

function Case_Statement_Range return String is
  type T is range 0 .. 10;
  X : T := 6;
begin
  case X is
    when 0 .. 5  => return "Zero to five";
    when 6 .. 10 => return "Six to ten";
    when others  => return "Invalid";
  end case;
end Case_Statement_Range;

This is also a problem for the case expression feature, which is supported by very similar code. @hannes-steffenhagen-diffblue mentioned that he might have a decent idea about how to solve this particular issue.