just use full-normalization when for the impl trait ref

This seems better because I want to avoid the situation where unresolved
inference variables make it into the environment.  On the other hand, I
am not 100% sure that this is correct. My assumption was that the WF
check should ensure that this normalization can succeed. But it occurs
to me that the WF checks may need to make use of the `specializes`
predicate themselves, and hence we may have a kind of cycle here (this
is a bigger problem with spec in any case that we need to resolve).

On the other hand, this should just cause extra errors I think, so it
seems like a safe thing to attempt. Certainly all tests pass.

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