union with void arm can generate invalid code

[bertschinger]

the following case:

enum Cases {
    one = 1,
    two = 2
};

union foo switch (Cases blah) {
case one:
        int a;
case two:
        void;
};

compiles to syntactically invalid Rust code under the zcopy API.

[bertschinger]

This case also generates code that doesn't compile:

enum Cases {
        one = 1,
        two = 2
};

union AnOption switch (bool yes) {
case TRUE:
    int a;
case FALSE:
    void;
};

union Foo switch(Cases blah) {
case one:
        AnOption a;
case two:
        void;
};