support for directCounterMap

[cc10512]

There is one other place where i saw this used. jsonconverter was looking it up in the map:

                } else if (eb->type->name == v1model.directCounter.name) {
                    auto it = directCountersMap.find(name);
                    if (it == directCountersMap.end()) {
                        ::warning("%1%: Direct counter not used; ignoring", c);
                    } else {

That went out with the different handling of externs i suppose. In any case, it looks like the backend was not generating code for the extern if it was not used. How are we handling this case?