Need to re-analyze method groups

[jcouv]

In Binder.CreateConversion, the bound method group node gets fixed to have a type (with oblivious annotations). During nullable analysis, we need to undo and re-do that with proper nullable annotation.

Note there is a parallel with lambda conversions. In NullableWalker.GetUnboundLambda() and NullableWalker.ApplyConversion we do this process for lambdas. I suspect we need something similar for method groups.

We should test all scenarios involving NullableWalker.RemoveConversion(). I suspect this logic of undoing effects of Binder.CreateConversion should be factored there. For example, new[] { new Func<string?>(...), () => "" };.

        [Fact]
        public void SuppressNullableWarning_LambdaInOverloadResolution()
        {
            var source =
@"class C
{
    static void Main(string? x)
    {
        var s = M(() => { return x; });
        s /*T:string?*/ .ToString(); // 1

        var s2 = M(() => { return x; }!); // suppressed
        s2 /*T:string?*/ .ToString(); // 2

        var s3 = M(M2);
        s3 /*T:string*/ .ToString(); // 3

        var s4 = M(M2!); // suppressed
        s4 /*T:string*/ .ToString(); // 4
    }
    static T M<T>(System.Func<T> x) => throw null;
    static string? M2() => throw null;
}";
            var comp = CreateCompilation(source, options: WithNonNullTypesTrue());
            comp.VerifyTypes();

            // TODO2
            // Missing warnings on s3 and s4
            comp.VerifyDiagnostics(
                // (6,9): warning CS8602: Possible dereference of a null reference.
                //         s /*T:string?*/ .ToString(); // 1
                Diagnostic(ErrorCode.WRN_NullReferenceReceiver, "s").WithLocation(6, 9),
                // (9,9): warning CS8602: Possible dereference of a null reference.
                //         s2 /*T:string?*/ .ToString(); // 2
                Diagnostic(ErrorCode.WRN_NullReferenceReceiver, "s2").WithLocation(9, 9)
                );
            CompileAndVerify(comp);
        }