Spurious clone for cast'ed fn ptrs

This:
```cpp
void f(int (*a)()) {
  (void)a;
}
```

Produces:
```rust
pub fn f_0(a: Ptr<i32>) {
    let a: Value<Ptr<i32>> = Rc::new(RefCell::new(a));
    (*a.borrow()).clone();   // <-- unneded clone
}
```