Trigger the standard autoloader as the last resort

As v2 isn't here yet, and looking at doctrine#232 it is unclear if will, and as we have to put up with annoying deprecated functions, here I propose to fall back to the standard autoloader if nothing else worked, and if there's no custom loader defined.

Fixes doctrine#270

lib/Doctrine/Common/Annotations/AnnotationRegistry.php

@@ -166,6 +166,10 @@ public static function loadAnnotationClass(string $class) : bool
                 return true;
             }
         }
+
+        if (self::$loaders === [] && self::$autoloadNamespaces === [] && \class_exists($class)) {
+            return true;
+        }
 
         self::$failedToAutoload[$class] = null;