privacy: Refactor top-level visiting in `TypePrivacyVisitor`
Full hierarchical visiting (`nested_filter::All`) is not necessary, visiting all item-likes in isolation is enough.
Tracking current item is not necessary, just keeping the current `mod` item is enough.
`visit_generic_arg` should behave like its default version, including checking types of const arguments.
Some comments, including FIXMEs, are also added.
Noticed while reading code to review https://github.com/rust-lang/rust/pull/113671.
r? ``@oli-obk``
fee8f00024