Fix phutil_analyzer bug on interface dependency creation

Summary:
Use $interface_name instead of $class_name when dealing with
interfaces. Problem happens when you extend an interface:

    interface SubIface extends SuperIface { ...

    Notice: Undefined variable: class_name in ...
	Fatal error: Call to a member function getConcreteString() on a
	non-object in ...

Test Plan: phutil_mapper.php works!

scripts/phutil_analyzer.php

@@ -303,7 +303,7 @@ foreach (Futures($futures) as $file => $future) {
       $extends = $interface->getChildByIndex(2);
       foreach ($extends->selectDescendantsOfType('n_CLASS_NAME') as $parent) {
         $requirements->addInterfaceDependency(
-          $class_name->getConcreteString(),
+          $interface_name->getConcreteString(),
           $parent,
           $parent->getConcreteString());
       }