perf: Cache already-checked types in the privacy visitor

[xmakro]

The privacy checker walks the full type of every expression and pattern in a module, re-walking the same type once per node it appears on. This caches the types and generic-args that walked clean (no privacy error) and skips them next time. A walk's result depends only on the interned value and the fixed module being checked, so a value that walks clean once walks clean everywhere.

Only clean walks are cached, so nothing is lost: a type that errors is never cached and still fires at every span, and no dep-graph edges are dropped since the full walk already ran once in the same query.

[petrochenkov]

Something similar was tried in #147486, but the caching overhead was found to be too large in most cases.

[petrochenkov]

@bors try @rust-timer queue

[rust-timer]

Awaiting bors try build completion.

@rustbot label: +S-waiting-on-perf

[petrochenkov]

@bors try @rust-timer queue

Needs a rebase first.

[rust-timer]

This pull request is already queued and waiting for a try build to finish.