[C# 7.3] Better (but not bestest) betterness fix

[BillWagner]

I found the following while working on #484

Consider this code:

public class BrokenOverloadResolution
{
    static void Test(Action a)
    {
    }

    static void Test(Func<object> f)
    {
    }

    public void MethodGroupExactMatching()
    {
        // This fails to compile with
        // 
        // error CS0121: The call is ambiguous between the following methods or properties: 'BrokenOverloadResolution.Test(Action)' and 'BrokenOverloadResolution.Test(Func<object>)'
        //
        // Fixed in 7.3
        // Probably in Method group conversion:
        // "For a method group conversion, candidate methods whose return type doesn't match up with the delegate's return type are removed from the set."
        // https://github.com/dotnet/roslyn/issues/8942#issuecomment-299536091
        //Test(Listen);

        //
        // non-void version works
        //
        // Test2 (DontListen);
    }

    void Listen()
    {
    }

    object DontListen()
    {
        return null;
    }
}

This compiles without error starting in V7.3, based on this issue: dotnet/roslyn#8942 (comment)

[jskeet]

Shorter version I'm using to check for breaking changes:

using System;

public class BrokenOverloadResolution
{
    static void Test(Action a)
    {
        Console.WriteLine("Action");
    }

    static void Test(Func<object> f)
    {
        Console.WriteLine("Func");
    }

    static void Main()
    {
        Test(DontListen);
    }

    static object DontListen()
    {
        return null;
    }
}

With C# compiler version 4.1.0-5.22109.6, this prints "Func"