Fix extra minus sign when formatting -0 on .NET Core 3.0

[Chicken-Bones]

.NET Core fixed some round trip formatting with floats https://devblogs.microsoft.com/dotnet/floating-point-parsing-and-formatting-improvements-in-net-core-3-0/

When decompiling -0f on .NET Core, ILSpy currently outputs --0f which is a compile error. Simplest reproducible test case:

using ICSharpCode.Decompiler;
using ICSharpCode.Decompiler.CSharp;
using ICSharpCode.Decompiler.CSharp.OutputVisitor;
using ICSharpCode.Decompiler.TypeSystem;
using System;
using System.Linq;
using System.Reflection;

namespace DecompilerTesting
{
    class Program
    {
        static void Main(string[] args)
        {
            var decompiler = new CSharpDecompiler(Assembly.GetExecutingAssembly().Location, new DecompilerSettings());
            var method = decompiler.TypeSystem.MainModule.GetTypeDefinition(new TopLevelTypeName("DecompilerTesting.Program")).GetMethods(m => m.Name == "MinusZero").Single();
            decompiler.Decompile(method.MetadataToken).AcceptVisitor(new CSharpOutputVisitor(Console.Out, FormattingOptionsFactory.CreateAllman()));
        }

        public static float MinusZero() => -0f;
    }
}