Add MaybeNullIfNullAttribute to round out nullable annotations

[mikernet]

One notably absent annotation missing is [MaybeNullIfNull] as the inverse to [NotNullIfNotNull]. One use-case would be to allow mimicking NVT lifted operator behavior with a reference type:

[return: MaybeNullIfNull(nameof(x))]
[return: MaybeNullIfNull(nameof(y))]
public static RefValue operator +(RefValue? x, RefValue? y)
{
    if (x is null || y is null)
        return null;

    // ...
}

[RikkiGibson]

I think the scenario might be addressed by dotnet/roslyn#43906