Dead code lint trips for publically exported struct variant fields

This shouldn't trip the lint since the variant is visible outside the crate but it is:

``` rust
#![feature(struct_variant)]
#![crate_type="lib"]

pub enum Foo {
    Bar {
        pub baz: int
    }
}
```

```
~ ❯ rustc test.rs
test.rs:6:13: 6:21 warning: code is never used: `baz`, #[warn(dead_code)] on by default
test.rs:6         pub baz: int
                      ^~~~~~~~
```
