Add a pass to mark doc attributes as used

The unused-attribute lint currently globally whitelists all doc attributes as used, even if they're placed in contexts that rustdoc would never look at (e.g. match arms). There should be a separate pass over the AST that marks the attributes in the correct contexts.

```rust
#![warn(unused_attributes)]

fn f() {
    match 10 {
        /// test
        _ => {}
    }
}
```
