ICE in new solver if we see an inference variable
By construction, we do not expect to see any `ty::Infer(ty::TyVar(_))` inference types in the solver (we treat this as ambiguous, since we need to be able to structurally resolve the self type at least one layer to assemble candidates for it). Additionally, since we're doing no freshening, we also don't expect to see any fresh vars of any kind in the solver.
Let's make that an ICE so we can catch any mistakes.
When #107282 lands, we should also ICE there too if we see a non-int/float infer.
r? `@lcnr`
