rustdoc: Render compiler built-in impls of "blessed" traits (in sections implementors & implemented traits)

[nagisa]

We have a bunch of traits that are normal traits but implemented automatically for a bunch of stuff. std::ptr::Pointee, std::marker::DiscriminantKind and soon std::enums::AsRepr (#89507).

rustdoc generated documentation fails to account for those and does not populate the trait's Implementors section: https://doc.rust-lang.org/stable/std/marker/trait.DiscriminantKind.html nor do the types that implement the trait present these traits in the implementations section (e.g. https://doc.rust-lang.org/stable/std/cmp/enum.Ordering.html#trait-implementations)

[camelid]

normal traits but implemented automatically

They're not "normal" though since at least for Pointee, the impls are hardcoded by the compiler IIUC.

[nagisa]

These all share that property, yes. By "normal" I really meant that they aren't an auto-trait.