deduplicate infer var instantiation
Having 3 separate implementations of one of the most subtle parts of our type system is not a good strategy if we want to maintain a sound type system ✨ while working on this I already found some subtle bugs in the existing code, so that's awesome 🎉 cc #121159
This was necessary as I am not confident in my nll changes in #119106, so I am first cleaning this up in a separate PR.
r? `@BoxyUwU`
we already use `instantiate_const_var`. This does lose some debugging
info for nll because we stop populating the `reg_var_to_origin` table with
`RegionCtxt::Existential(None)`, I don't think that matters however.
Supporting this adds additional complexity to one of the most involved
parts of the type system, so I really don't think it's worth it.
