Allow a ref field to refer to a ref struct type

Currently, `ref` fields cannot refer to `ref struct` types.

```csharp
ref struct R1<T>
{
}

ref struct R2<T>
{
    public ref R1<T> F; // error CS9050: A ref field cannot refer to a ref struct.
}
```

See https://github.com/dotnet/roslyn/pull/62186

See also https://github.com/dotnet/roslyn/issues/62098