Detect implicit conversion of async delegates

To complement [CS4014](https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/compiler-messages/cs4014) that detects `async` calls not `await`'ed, it could be nice to have an analyzer to detect implicit conversions from async delegates to sync ones.

```csharp
async Task FooAsync(Func func)
{
await Task.Delay(0);
func(); // the async delegate is not await'ed!
}

Task BarAsync(Func func)
{
return FooAsync(func); // implicit conversion to Func
}
```

It's not always easily detectable using Intellisense