Avoid sorting by DefId for `necessary_variants()`
Follow-up to https://github.com/rust-lang/rust/pull/83074. Originally I tried removing `impl Ord for DefId` but that hit *lots* of errors 😅 so I thought I would start with easy things.
I am not sure whether this could actually cause invalid query results, but this is used from `MarkSymbolVisitor::visit_arm` so it's at least feasible.
r? `@Aaron1011`
cb17136405