Support `rustc_confusables` on items other than methods

[estebank]

As a follow up #120730, #[doc(alias = "name")] and #[rustc_confusables(..)] should be supported on all items, so that the following

#[doc(alias = "plus")]
fn add(a: i32, b: i32) -> i32 { a + b }

fn main() {
    let _ = plus(1, 2);
}

the resolve error for plus should mention add as well.