Add support for variance safety for static interface members

[RexJaeschke]

I just discovered this V9 feature spec here. To make sure this was actually implemented in V9, I compiled the following code:

using System;
using System.Threading.Tasks;

public interface I<out T>
{
    static Task<T> F = Task.FromResult(default(T)); // No problem
    static Task<T> P => Task.FromResult(default(T));   //CS8904 in V8 but OK in V9
    static Task<T> M() => Task.FromResult(default(T));    //CS8904 in V8 but OK in V9
    static event EventHandler<T>? E; // CS8904 in V8 but OK in V9
}

The errors produced by V8 do not occur in V9.