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
fixesrust-lang/rust#121429
0b65d0db5f