Use a better `hashCode` for the result of `CaseFormat.converterTo`.

Using XOR here meant that the `hashCode` of `A.converterTo(B)` was the same as the `hashCode` of `B.converterTo(A)`. (It also meant that the `hashCode` of `A.converterTo(A)` was 0, though that probably doesn't often matter).

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PiperOrigin-RevId: 839820133