C++/C#: Move the Windows autobuilder into a subfolder in ql/csharp.

This is a necessary preparation for moving the C# dependency management to `paket`,
which in turn is a necessary preparation for moving the C# build to bazel.

As we discovered in https://github.com/github/codeql/pull/16376,
`paket` tries to restore all projects recursively from the root folder.
If we support building C# code under both `ql/csharp` and `ql/cpp`, we need
to have a single lockfile under `ql`, as both codebases share the same set of dependencies
(and utilities from `ql/csharp/extractor`).
Then, `paket` will also try to restore things that look like "C# projects" in other languages'
folders, which is not what we want.
Therefore, we address this by moving all C# code into a common root directory, `ql/csharp`.

This needs an internal PR to adjust the buildsystem to look for the autobuilder in the new location.

csharp/autobuilder/Semmle.Autobuild.Cpp/CppAutobuilder.cs

@@ -0,0 +1,38 @@
+using Semmle.Autobuild.Shared;
+using Semmle.Util;
+
+namespace Semmle.Autobuild.Cpp
+{
+    /// <summary>
+    /// Encapsulates C++ build options.
+    /// </summary>
+    public class CppAutobuildOptions : AutobuildOptionsShared
+    {
+        public override Language Language => Language.Cpp;
+
+
+        /// <summary>
+        /// Reads options from environment variables.
+        /// Throws ArgumentOutOfRangeException for invalid arguments.
+        /// </summary>
+        public CppAutobuildOptions(IBuildActions actions) : base(actions)
+        {
+        }
+    }
+
+    public class CppAutobuilder : Autobuilder<CppAutobuildOptions>
+    {
+        public CppAutobuilder(IBuildActions actions, CppAutobuildOptions options) : base(actions, options, new DiagnosticClassifier()) { }
+
+        public override BuildScript GetBuildScript()
+        {
+            return
+                // First try MSBuild
+                new MsBuildRule().Analyse(this, true) |
+                // Then look for a script that might be a build script
+                (() => new BuildCommandAutoRule((_, f) => f(null)).Analyse(this, true)) |
+                // All attempts failed: print message
+                AutobuildFailure();
+        }
+    }
+}

csharp/autobuilder/Semmle.Autobuild.Cpp/Program.cs

@@ -0,0 +1,35 @@
+using System;
+
+using Semmle.Autobuild.Shared;
+using Semmle.Util;
+
+namespace Semmle.Autobuild.Cpp
+{
+    class Program
+    {
+        static int Main()
+        {
+
+            try
+            {
+                var actions = SystemBuildActions.Instance;
+                var options = new CppAutobuildOptions(actions);
+                try
+                {
+                    Console.WriteLine("CodeQL C++ autobuilder");
+                    using var builder = new CppAutobuilder(actions, options);
+                    return builder.AttemptBuild();
+                }
+                catch (InvalidEnvironmentException ex)
+                {
+                    Console.WriteLine("The environment is invalid: {0}", ex.Message);
+                }
+            }
+            catch (ArgumentOutOfRangeException ex)
+            {
+                Console.WriteLine("The value \"{0}\" for parameter \"{1}\" is invalid", ex.ActualValue, ex.ParamName);
+            }
+            return 1;
+        }
+    }
+}

csharp/autobuilder/Semmle.Autobuild.Cpp/Properties/AssemblyInfo.cs

@@ -0,0 +1,32 @@
+using System.Reflection;
+using System.Runtime.InteropServices;
+
+// General Information about an assembly is controlled through the following
+// set of attributes. Change these attribute values to modify the information
+// associated with an assembly.
+[assembly: AssemblyTitle("Semmle.Autobuild.Cpp")]
+[assembly: AssemblyDescription("")]
+[assembly: AssemblyConfiguration("")]
+[assembly: AssemblyCompany("GitHub")]
+[assembly: AssemblyProduct("CodeQL autobuilder for C++")]
+[assembly: AssemblyCopyright("Copyright © GitHub 2020")]
+[assembly: AssemblyTrademark("")]
+[assembly: AssemblyCulture("")]
+
+// Setting ComVisible to false makes the types in this assembly not visible
+// to COM components.  If you need to access a type in this assembly from
+// COM, set the ComVisible attribute to true on that type.
+[assembly: ComVisible(false)]
+
+// Version information for an assembly consists of the following four values:
+//
+//      Major Version
+//      Minor Version
+//      Build Number
+//      Revision
+//
+// You can specify all the values or you can default the Build and Revision Numbers
+// by using the '*' as shown below:
+// [assembly: AssemblyVersion("1.0.*")]
+[assembly: AssemblyVersion("1.0.0.0")]
+[assembly: AssemblyFileVersion("1.0.0.0")]

csharp/autobuilder/Semmle.Autobuild.Cpp/Semmle.Autobuild.Cpp.csproj

@@ -0,0 +1,28 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net8.0</TargetFramework>
+    <AssemblyName>Semmle.Autobuild.Cpp</AssemblyName>
+    <RootNamespace>Semmle.Autobuild.Cpp</RootNamespace>
+    <ApplicationIcon />
+    <OutputType>Exe</OutputType>
+    <StartupObject />
+    <GenerateAssemblyInfo>false</GenerateAssemblyInfo>
+    <RuntimeIdentifiers>win-x64;linux-x64;osx-x64</RuntimeIdentifiers>
+    <Nullable>enable</Nullable>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <Folder Include="Properties\" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Microsoft.Build" Version="17.8.3" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\..\extractor\Semmle.Util\Semmle.Util.csproj" />
+    <ProjectReference Include="..\..\autobuilder\Semmle.Autobuild.Shared\Semmle.Autobuild.Shared.csproj" />
+  </ItemGroup>
+
+</Project>