Flag Task.Delay(0) as being Task.CompletedTask

Task.Delay(0) is the same as Task.CompletedTask
[.NET Core](https://source.dot.net/#System.Private.CoreLib/shared/System/Threading/Tasks/Task.cs,5396)
[.NET Framework](https://referencesource.microsoft.com/#mscorlib/system/threading/Tasks/Task.cs,5881)

Test cases:
```
Task.Delay(0);
Task.Delay(0, token);
```

Suggest:
We should suggest changing to `Task.CompletedTask` since that is what is going to happen anyway.

Read the linked .NET Core and .NET Framework Task implementations, then use the two listed Task.Delay cases as the behavior to verify. Locate the repository's diagnostic entry point and ensure both cases are flagged with a suggestion to use Task.CompletedTask; done means the diagnostics and their tests cover both forms.

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