[swift-generator] skip un-generate-able constants

There is a bug that the swift generator cannot handle certain kinds of constants (e.g. maps that have enums for key or value type). This bug will take a bit of restructuring in the generator to fix properly, but some of the users of the generator don't actually need those constants generated at this point, they just need the generator to keep going instead of break and fail to generate any further types. This change does that.