fix logic error in rust-lang#44269's prune_cache_value_obligations

We want to retain obligations that *contain* inference variables, not
obligations that *don't contain* them, in order to fix rust-lang#43132. Because
of surrounding changes to inference, the ICE doesn't occur in its
original case, but I believe it could still be made to occur on master.

Maybe I should try to write a new test case? Certainly not right now
(I'm mainly trying to get us a beta that we can ship) but maybe before
we land this PR on nightly?

This seems to cause a 10% performance regression in my imprecise
attempt to benchmark item-body checking for rust-lang#43613, but it's better to
be slow and right than fast and wrong. If we want to recover that, I
think we can change the constrained-type-parameter code to actually
give a list of projections that are important for resolving inference
variables and filter everything else out.

Author: arielb1

src/librustc/traits/project.rs

@@ -604,7 +604,7 @@ fn prune_cache_value_obligations<'a, 'gcx, 'tcx>(infcx: &'a InferCtxt<'a, 'gcx,
                   // but we have `T: Foo<X = ?1>` and `?1: Bar<X =
                   // ?0>`).
                   ty::Predicate::Projection(ref data) =>
-                      !infcx.any_unresolved_type_vars(&data.ty()),
+                      infcx.any_unresolved_type_vars(&data.ty()),
 
                   // We are only interested in `T: Foo<X = U>` predicates, whre
                   // `U` references one of `unresolved_type_vars`. =)