Allow specifying an IDecompilerTypeSystem rather than a DecompilerTypeSystem to CSharpDecompiler

ICSharpCode.Decompiler/CSharp/CSharpDecompiler.cs

@@ -257,7 +257,7 @@ namespace ICSharpCode.Decompiler.CSharp
 		/// <summary>
 		/// Creates a new <see cref="CSharpDecompiler"/> instance from the given <paramref name="typeSystem"/> and the given <paramref name="settings"/>.
 		/// </summary>
-		public CSharpDecompiler(DecompilerTypeSystem typeSystem, DecompilerSettings settings)
+		public CSharpDecompiler(IDecompilerTypeSystem typeSystem, DecompilerSettings settings)
 		{
 			this.typeSystem = typeSystem ?? throw new ArgumentNullException(nameof(typeSystem));
 			this.settings = settings;