Fix nonempty variadic parameters declared in body

For nonempty variadic parameters that were declared in the body, like this:

    void f(elems) {
        String+ elems;
        // ...
    }

the `TypeVisitor` forgot to carry over the `atLeastOne` information from the `SequencedType`. This blew up the Java backend (incompatible type, `Sequential` vs `Sequence`). (The JS backend didn’t care.)