fix logic error in #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 #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 #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.

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