rust-lang#58972 ignored extern_traits because before rust-lang#65983 was fixed, they
would always fail to resolve, giving spurious warnings.
This undoes that change, so extern traits are now seen by the
`collect_intra_doc_links` pass. There are also some minor changes in
librustdoc/fold.rs to avoid borrowing the extern_traits RefCell more
than once at a time.

…olution

This avoids a rare rustdoc bug where loading `core` twice caused a
'cannot find a built-in macro' error:

1. `x.py build --stage 1` builds the standard library and creates a sysroot
2. `cargo doc` does something like `cargo check` to create `rmeta`s for all the crates (unrelated to what was built above)
3. the `cargo check`-like `libcore-*.rmeta` is loaded as a transitive dependency *and claims ownership* of builtin macros
4. `rustdoc` later tries to resolve some path in a doc link
5. suggestion logic fires and loads "extern prelude" crates by name
6. the sysroot `libcore-*.rlib` is loaded and *fails to claim ownership* of builtin macros

This fixes step 5. by not running suggestion logic if this is a
speculative resolution. Additionally, it marks `resolve_ast_path` as a
speculative resolution.