Fix class name in generated class file. (#20961)

* Fix class name in generated class file.

If a class name used in a make commands starts with a slash the generated file will have the slash in the class name creating syntax error in class file. For example, artisan make modeld command (i.e. php artisan make:model \Foo) will generate file with the class name \Foo (i.e. class \Foo { ..... } ).  Removing the beginning slash in the GeneratorCommand::qualifyClass() method will generate file will a valid class name.

* Update GeneratorCommand.php

src/Illuminate/Console/GeneratorCommand.php

@@ -80,6 +80,8 @@ public function handle()
      */
     protected function qualifyClass($name)
     {
+        $name = ltrim($name, '\\/');
+
         $rootNamespace = $this->rootNamespace();
 
         if (Str::startsWith($name, $rootNamespace)) {