Building on Bertrand's answer at https://stackoverflow.com/a/16069782/492 - make a console app out of this and run it in the root folder where you want the VS 2015 Solution to appear. It works for C# & VB (hey! be nice).
It overwrites anything existing but you source control, right?
Check a recently used .SLN file to see what the first few writer.WriteLine()  header lines should actually be by the time you read this.
Don't worry about the project type GUID Ptoject("0") - Visual Studio will work that out and write it in when you save the .sln file.
using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text;
using System.Xml;
using System.Xml.Linq;
namespace AddAllProjectsToNewSolution
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("starting");
            using (var writer = new StreamWriter("AllProjects.sln", false, Encoding.UTF8))
            {
                writer.WriteLine("Microsoft Visual Studio Solution File, Format Version 14.00");
                writer.WriteLine("# Visual Studio 14");
                writer.WriteLine("VisualStudioVersion = 14.0.25420.1");
                var seenElements = new HashSet<string>();
                foreach (var file in (new DirectoryInfo(Directory.GetCurrentDirectory())).GetFiles("*.*proj", SearchOption.AllDirectories))
                {
                    string extension = file.Extension;
                    if (extension != ".csproj" && extension != ".vbproj")
                    {
                        Console.WriteLine($"ignored {file.Name}");
                        continue;
                    }
                    Console.WriteLine($"adding {file.Name}");
                    string fileName = Path.GetFileNameWithoutExtension(file.Name);
                    if (seenElements.Add(fileName))
                    {
                        var guid = ReadGuid(file.FullName);
                        writer.WriteLine($"Project(\"0\") = \"{fileName}\", \"{GetRelativePath(file.FullName)} \", \"{{{guid}}}\"" );
                        writer.WriteLine("EndProject");
                    }
                } 
            }
             Console.WriteLine("Created AllProjects.sln. Any key to close");
             Console.ReadLine();
        }
        static Guid ReadGuid(string fileName)
        {
            using (var file = File.OpenRead(fileName))
            {
                var elements = XElement.Load(XmlReader.Create(file));
                return Guid.Parse(elements.Descendants().First(element => element.Name.LocalName == "ProjectGuid").Value);
            }
        }
        // https://stackoverflow.com/a/703292/492
        static string GetRelativePath(string filespec, string folder = null)
        {
            if (folder == null)
                folder = Environment.CurrentDirectory;
            Uri pathUri = new Uri(filespec);
            // Folders must end in a slash
            if (!folder.EndsWith(Path.DirectorySeparatorChar.ToString()))
                folder += Path.DirectorySeparatorChar;
            Uri folderUri = new Uri(folder);
            return Uri.UnescapeDataString(folderUri.MakeRelativeUri(pathUri).ToString().Replace('/', Path.DirectorySeparatorChar));
        }
    }
}