[Executors] Fix delegation chain of Excecutor.enqueue for Job specifically #65732
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Resolves: rdar://108788736
Since we now have THREE overloads of enqueue and must allow implementing just ONE of them, the delegation pattern must form a ring:
Unowned Job -> Executor Job -> Job -> ---||---
we didn't do this and delegated to UnownedJob based on directly from ExecutorJob which would have caused an infinite cycle if only (deprecated)Job
based enqueue was implemented. We issue warnings in typechecking and require that one of these methods will be implemented already, so the "cyclic delegation" is always safe -- at least one of the methods will break the cycle by having an end user implement it.