dead code lint considers #[used] items unused

To reproduce (a more realistic example would use `#[link_section]`):
```rust
#![feature(used)]
#[used]
static FOO: u32 = 0;
fn main(){}
```
```
warning: static item is never used: `FOO`
 --> <anon>:3:1
  |
3 | static FOO: u32 = 0;
  | ^^^^^^^^^^^^^^^^^^^^
  |
  = note: #[warn(dead_code)] on by default
```