[gcjx] Generalise visitor

[Tom Tromey]

>>>>> "Ranjit" == Ranjit Mathew <rmathew@gmail.com> writes:

>> Ideally model_element::visit would be abstract.  Presumably there are
>> still other elements that aren't handled?

Ranjit> Yes, that's how I had started. But parse::compilation_unit()
Ranjit> actually creates an object of type model_element.

What an ugly hack!

Actually, if we want threading to work properly, we may need to change
this particular area anyway.  Currently it is building a model_package
during parsing, which is a no-no if we are parsing and analyzing at
the same time; instead a "package forwarder" element would be
appropriate.

Tom