Auto merge of #16039 - WaffleLapkin:don't-emit-missing-assoc-items-diagnostic-for-negative-impls, r=Veykril

fix: Don't emit "missing items" diagnostic for negative impls

Negative impls can't have items, so there is no reason for this diagnostic.

LMK if I should add a test somewhere. Also LMK if that's not how we usually check multiple things in an if in r-a.