Auto merge of #56878 - petrochenkov:privdyn, r=arielb1

privacy: Use common `DefId` visiting infrastructure for all privacy visitors

One repeating pattern in privacy checking is going through a type, visiting all `DefId`s inside it and doing something with them.
This is the case because visibilities and reachabilities are attached to `DefId`s.

Previously various privacy visitors visited types slightly differently using their own methods, with most recently written `TypePrivacyVisitor` being the "gold standard".
This mostly worked okay, but differences could manifest in overly conservative reachability analysis, some errors being reported twice, some private-in-public lints (not errors) being wrongly reported or not reported.

This PR does something that I wanted to do since https://github.com/rust-lang/rust/pull/32674#discussion_r58291608 - factoring out the common visiting logic!
Now all the common logic is contained in `struct DefIdVisitorSkeleton`, with specific privacy visitors deciding only what to do with visited `DefId`s (via `trait DefIdVisitor`).

A bunch of cleanups is also applied in the process.
This area is somewhat tricky due to lots of easily miss-able details, but thankfully it's was well covered by tests in https://github.com/rust-lang/rust/pull/46083 and previous PRs, so I'm relatively sure in the refactoring correctness.

Fixes https://github.com/rust-lang/rust/pull/56837#discussion_r241962239 in particular.
Also this will help with implementing https://github.com/rust-lang/rust/issues/48054.

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