Auto merge of #118120 - compiler-errors:closure-kind, r=lcnr

Remove `PredicateKind::ClosureKind`

We don't need the `ClosureKind` predicate kind -- instead, `Fn`-family trait goals are left as ambiguous, and we only need to make progress on `FnOnce` projection goals for inference purposes.

This is similar to how we do confirmation of `Fn`-family trait and projection goals in the new trait solver, which also doesn't use the `ClosureKind` predicate.

Some hacky logic is added in the second commit so that we can keep the error messages the same.

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