Fix `trait_ref_to_existential` as trait aliases is stabilized

[estebank]

#66392 (comment)

There appears to be a missing filter on top of expand_trait_aliases, which picks up non-supertraits where clauses - but also, the object safety completely ignores trait aliases, which could be object safety hazards. We now use delay_span_bug there to avoid an ICE in stable even when the feature is disabled.