UseAutoProperty generates invalid code when using ref on the field

```c#
class Class
{
    int value;
    int Value
    {
        get { return value; }
    }

    void M()
    {
        ref int x = ref value;
    }
}
```

applying the code fix results in:
```c#
class Class
{
    int Value { get; set; }
    
    void M()
    {
        ref int x = ref Value;
    }
}
```
which doesn't compile because Value doesn't return by ref.