Dead code lint warns on types used in generics

``` Rust
struct A;

fn f<T>() {}

fn main() {
    f::<A>();
    let _b = std::mem::size_of::<A>();
}
```

```
t.rs:1:0: 1:9 warning: code is never used: `A`, #[warn(dead_code)] on by default
t.rs:1 struct A;
       ^~~~~~~~~
```

cc @ktt3ja
