soundness holes in let

Related to #3235 because the same holes don't exist in `match`.

``` rust
struct Foo {
    n: uint
}

impl Drop for Foo {
    fn finalize(&self) {
        for self.n.times {
            println("pew");
        }
    }
}

fn main() {
    let x = Foo{n: 5};
    let &_y = &x;
}
```
