Don't elaborate non-obligations into obligations
It's suspicious to elaborate a `PolyTraitRef` or `Predicate` into an `Obligation`, since the former does not have a param-env associated with it, but the latter does. This is a footgun that, while not being misused *currently* in the compiler, easily could be misused by someone less familiar with the elaborator's inner workings.
This PR just changes the API -- ideally, the elaborator wouldn't even have to deal with obligations if we're not elaborating obligations, but that would require a bit more abstraction than I could be bothered with today.
6535e66fa5