Delay replacing escaping bound vars in `FindParamInClause`
By uplifting the `BoundVarReplacer`, which is used by (e.g.) normalization to replace escaping bound vars that are encountered when folding binders, we can use a similar strategy to delay the instantiation of a binder's contents in the `FindParamInClause` used by the new trait solver.
This should alleviate the recently added requirement that `Binder<T>: TypeVisitable` only if `T: TypeFoldable`, which was previously required b/c we were calling `enter_forall` so that we could structurally normalize aliases that we found within the predicates of a param-env clause.
r? lcnr
c3537c2f9e