Auto merge of #142074 - oli-obk:its-finally-gone, r=petrochenkov

Remove CollectItemTypesVisitor

I always felt like we were very unnecessarily walking the HIR, let's see if perf agrees

There is lots to ~~improve~~ consolidate further here, as we still have 3 item wfchecks:

* check_item (matching on the hir::ItemKind)
    * actually doing trait solver based checks (by using HIR spans)
* lower_item (matching on the hir::ItemKind after loading it again??)
    * just ensure_ok-ing a bunch of queries
* check_item_type (matching on DefKind)
    * some type based checks, mostly ensure_ok-ing a bunch of queries

fixes rust-lang/rust#121429

Author: bors