Auto merge of #75176 - jyn514:impl-link, r=GuillaumeGomez,petrochenkov

Fix intra-doc links for cross-crate re-exports of default trait methods

The original fix for this was very simple: https://github.com/rust-lang/rust/pull/58972 ignored `extern_traits` because before https://github.com/rust-lang/rust/issues/65983 was fixed, they would always fail to resolve, giving spurious warnings. So the first commit just 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.

However, that brought up a much more thorny problem. `rustc_resolve` started giving 'error: cannot find a built-in macro with name `cfg`' when documenting `libproc_macro` (I still haven't been able to reproduce on anything smaller than the full standard library). The chain of events looked like this (thanks @eddyb for the help debugging!):

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

`rustc_resolve` gives the error after step 5. However, `rustdoc` doesn't need suggestions at all - `resolve_str_path_error` completely discards the `ResolutionError`! The fix implemented in this PR is to skip the suggestion logic for `resolve_ast_path`: pass `record_used: false` and skip `lookup_import_candidates` when `record_used` isn't set.

It's possible that if/when https://github.com/rust-lang/rust/issues/74207 is implemented this will need a more in-depth fix which returns a `ResolutionError` from `compile_macro`, to allow rustdoc to reuse the suggestions from rustc_resolve. However, that's a much larger change and there's no need for it yet, so I haven't implemented it here.

Fixes https://github.com/rust-lang/rust/issues/73829.

r? @GuillaumeGomez

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