Process derives before other attributes

[dtolnay]

Moved from https://github.com/serde-rs/serde/issues/612.

#[serde(deny_unknown_fields)]
#[derive(Deserialize)]
pub struct SomeStruct {
    /* ... */
}

This works in Macros 1.1 expansion but not with Syntex. Macros 1.1 expands derives ahead of other attributes and we should do the same.