[ty] Intern `InferableTypeVars` (#24161)
Most importantly, this lets us use `InferableTypeVars` as the parameter
to a salsa-tracked function, which, try as I might, I end up needing for
the `SpecializationBuilder` refactoring.
Plus, the type was over-thought to begin with. By far most of these are
created by querying a `GenericContext`, via a method that was already
salsa-tracked. The main thing that we lose is that, when checking
assignability of generic callables, we used to be able to "combine" the
various inferable sets without having to actually construct a new hash
set. Now we do have to do that. But, we can salsa-track the `merge`
method, which should hopefully claw back some of that.
