[generator] Do the right thing when [ReleaseAttribute] is applied to a member that returns a managed array

While updating [NetworkExtension](https://github.com/xamarin/xamarin-macios/pull/8126#discussion_r393570319) we uncovered a generator bug that produces incorrect code:

```csharp
[return: Release]
[return: NullAllowed]
[Export ("copyAppRules")]
NEAppRule[] CopyAppRules ();
```

which generates:

```csharp
[Export ("copyAppRules")]
[return: ReleaseAttribute ()]
[BindingImpl (BindingImplOptions.GeneratedCode | BindingImplOptions.Optimizable)]
public virtual NEAppRule[] CopyAppRules ()
{
NEAppRule[] ret;
if (IsDirectBinding) {
ret = NSArray.ArrayFromHandle(global::ObjCRuntime.Messaging.IntPtr_objc_msgSend (this.Handle, Selector.GetHandle ("copyAppRules")));
} else {
ret = NSArray.ArrayFromHandle(global::ObjCRuntime.Messaging.IntPtr_objc_msgSendSuper (this.SuperHandle, Selector.GetHandle ("copyAppRules")));
}
if (ret != null)
global::ObjCRuntime.Messaging.void_objc_msgSend (ret.Handle, Selector.GetHandle ("release"));
return ret;
}
```

`ret` will not have the `Handle` property at this point since it is already too late it is a managed array.