Dead code lint shouldn't warn about #[static_assert] statics

``` rust
#[static_assert]
static X: bool = true;

fn main() {}
```

```
static-assert.rs:2:1: 2:23 warning: code is never used: `X`, #[warn(dead_code)] on by default
static-assert.rs:2 static X: bool = true;
                   ^~~~~~~~~~~~~~~~~~~~~~
```

These might not be used directly in the rest of the code, but they are still serving a concrete purpose, i.e. they are being semantically used.
