Track nullable assignment through ?ref:ref receiver

[gafter]

At the LDM on 2019-02-13, we agreed on our intent to track assignments through ref expressions (though we will not track aliasing through ref variables). The following would be a good test. We currently report the third and fourth expected warnings, but we fail to report the first and second.

#nullable enable
    static void M(bool b, string nonNull, string? maybeNull)
    {
        string? x = nonNull;
        string? y = nonNull;
        (b ? ref x : ref y) = maybeNull;
        x.ToString(); // should be warning: possible dereference of a null reference
        y.ToString(); // should be warning: possible dereference of a null reference

        x = maybeNull;
        y = maybeNull;
        (b ? ref x : ref y) = nonNull;
        x.ToString(); // should be warning: possible dereference of a null reference
        y.ToString(); // should be warning: possible dereference of a null reference
    }

[jcouv]

Moved to post C# 8 RTM.