Nullable semantic model mishandles argument conversion types

[333fred]

See IdentityConversion_NullCoalescingOepration_02. ApplyArgumentConversion is applying the result of the conversion to the underlying BoundNullCoalescingOperator, which isn't what was intended. Original text of the affected part of the test is below.

    static void F1(IIn<object>? x1, IIn<object?>? y1)
    {
        FIn((x1 ?? y1)/*T:IIn<object!>?*/); // This is returning IIn<object?>?
        FIn((y1 ?? x1)/*T:IIn<object!>?*/); // This is returning IIn<object?>?
    }

    static void F3(object? x3, object? y3)
    {
        FIn((FIn(x3) ?? FIn(y3))/*T:IIn<object?>?*/); // A
        if (x3 == null) return;
        FIn((FIn(x3) ?? FIn(y3))/*T:IIn<object!>?*/); // B
        FIn((FIn(y3) ?? FIn(x3))/*T:IIn<object!>?*/); // C
        if (y3 == null) return;
        FIn((FIn(x3) ?? FIn(y3))/*T:IIn<object!>?*/); // D
    }