intra-rustdoc links fail on primitive methods

[tspiteri]

I hit this in #58596.

For example, for this a.rs

//! [`Vec`] is fine.
//! [`Vec::len`] is fine.
//! [`char`] is fine.
//! [`char::len_utf8`] is not.

rustdoc +nightly a.rs produces

 --> a.rs:4:6
  |
4 | //! [`char::len_utf8`] is not.
  |      ^^^^^^^^^^^^^^^^ cannot be resolved, ignoring
  |
  = note: #[warn(intra_doc_link_resolution_failure)] on by default
  = help: to escape `[` and `]` characters, just add '\' before them like `\[` or `\]`