Fixes #153477.

This is the other half of #99698, which fixed the case where the parent module is #[doc(hidden)] but left the case where the re-export itself is #[doc(hidden)] as a FIXME (with a tracking test in dont-suggest-doc-hidden-variant-for-enum/hidden-child.rs).

The problem: when a crate does #[doc(hidden)] pub use core::error::Error, diagnostics pick up the hidden re-export path instead of the canonical one. For example, snafu::Error instead of std::error::Error.

Two changes:

In visible_parent_map, the add_child closure now checks whether the re-export itself is #[doc(hidden)] via reexport_chain and sends it to fallback_map, same treatment as doc-hidden parents and underscore re-exports.

should_encode_attrs now returns true for DefKind::Use. Without this, #[doc(hidden)] on use items was never written to crate metadata, so is_doc_hidden always returned false cross-crate. This was the actual root cause, the check in visible_parent_map alone isn't enough if the attribute isn't in the metadata.

The existing FIXME test now serves as the regression test. The .stderr goes from suggesting hidden_child::__private::Some(1i32) to just Some(1i32).

cc @eggyal