Emit a more specific error when using the null suppression operator on a Nullable<T>

[JoeRobich]

The current error isn't useful in leading developers to using name.Value instead.

#nullable enable

class Program
{
    static void Main(string[] args)
    {
        if (TryGetName(out var name))
        {
            SayHello(name!);
            //          ^--- CS1503: cannot convert from 'Name?' to 'Name'
        }
    }

    static bool TryGetName([NotNullWhen(returnValue: true)]out Name? name)
    {
        name = Name.Jon;
        return true;
    }

    static void SayHello(Name name)
    {
        Console.WriteLine(name);
    }
}

enum Name
{
    Jon,
    Bob
}

[jcouv]

The error message is accurate.
The ! suppression operator doesn't do any conversion, and so doesn't affect the error that we produce when assigning Nullable<Name> to Name.