[rustdoc] Correctly handle rustc_allow_incoherent_impl on primitive methods

[GuillaumeGomez]

Needed for #162808.
First commit comes from #163133.

This is more of a hack than a proper fix as I don't think there is a better way to handle this. So in short:

• If a primitive incoherent impl method is implemented outside of core/alloc/std, we don't link to it.
• If in alloc/std, we link to std

r? @Urgau

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.