rustdoc: Make sure that items are not constructed multiple times during doc inlining

[petrochenkov]

In #96282 (comment) there was some suspicion that it may happen.

By making sure that items, and Attributes in particular, are not constructed multiple times we may improve rustdoc's performance and uncover some logical bugs.

[GuillaumeGomez]

I added a static hashset in clean/types.rs with DefId as key and then this code in Item::from_def_id_and_attrs_and_parts:

if !HASHMAP.lock().unwrap().insert(def_id) {
    eprintln!("+> {:?}", def_id);
}

Well, a LOT of duplicates coming from blanket impls code apparently.