Rollup merge of #135697 - compiler-errors:poly-trait-ref, r=lqd

Get rid of `ToPolyTraitRef`

It's generally a footgun, since it throws away `PredicatePolarity`.

This PR doesn't attempt to fix any related bugs having to do with binders or polarity; it just tries to pass through `TraitPredicate`s around instead of `TraitRef`s. There should be basically no functional changes.