Fixes rust-lang#50159

This commit makes several improvements to AutoTraitFinder:

* Call infcx.resolve_type_vars_if_possible before processing new
predicates. This ensures that we eliminate inference variables wherever
possible.
* Process all nested obligations we get from a vtable, not just ones
with depth=1.
  * The 'depth=1' check was a hack to work around issues processing
certain predicates. The other changes in this commit allow us to
properly process all predicates that we encounter, so the check is no
longer necessary,
* Ensure that we only display predicates *without* inference variables
to the user, and only attempt to unify predicates that *have* an
inference variable as their type.

Additionally, the internal helper method is_of_param now operates
directly on a type, rather than taking a Substs. This allows us to use
the 'self_ty' method, rather than directly dealing with Substs.

If we end up with a projection predicate that equates a type with
itself (e.g. <T as MyType>::Value == <T as MyType>::Value), we can
run into issues if we try to add it to our ParamEnv.