"Deconstruct variable declaration" introduces error.

Start with code:

```c#
        (int x, int y) t = default;
        Console.WriteLine(t.x + t.y);
```

Use 'Deconstruct variable declaration' on 't'.  It produces the following code:

```c#
        (int x, int y) = default;
        Console.WriteLine(x + y);
```

This is not allowed because 'default' is not deconstructable to a tuple.  