If a user-defined type has `AbslStringify()` defined, it will always be used for logging over `operator<<`.

`HasAbslStringify` now uses the empty class `UnimplementedSink` for its checks instead of `StringifySink` in order to make it work in cases involving other sinks.

PiperOrigin-RevId: 485710377
Change-Id: Ibdd916151c7abc3269c35fbe79b772867f3d25e1