Improve useless overriding method linter rule

Summary:
Improve `ArcanistUselessOverridingMethodXHPASTLinterRule` by allowing overriding methods which set default values. For example, the following scenario is perfectly valid:

```lang=php

class SomeClass {
  public function __construct($x) {}
}

class SomeOtherClass extends Class {
  public function __construct($x = null) {
    parent::__construct($x);
  }
}
```

Test Plan: Added test case.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D13840

src/lint/linter/__tests__/xhpast/useless-overriding-method.lint-test

@@ -16,6 +16,10 @@ final class MyClass extends SomeOtherClass {
   public function anotherMethod($x, &$y) {
     return parent::anotherMethod($x, $y);
   }
+
+  public function usefulMethodWithDefaultValue($x = null) {
+    return parent::usefulMethodWithDefaultValue($x);
+  }
 }
 ~~~~~~~~~~
 error:3:13

src/lint/linter/xhpast/rules/ArcanistUselessOverridingMethodXHPASTLinterRule.php

@@ -26,12 +26,17 @@ final class ArcanistUselessOverridingMethodXHPASTLinterRule
       $parameters = array();
 
       foreach ($parameter_list->getChildren() as $parameter) {
+        $default   = $parameter->getChildByIndex(2);
         $parameter = $parameter->getChildByIndex(1);
 
         if ($parameter->getTypeName() == 'n_VARIABLE_REFERENCE') {
           $parameter = $parameter->getChildOfType(0, 'n_VARIABLE');
         }
 
+        if ($default->getTypeName() != 'n_EMPTY') {
+          continue 2;
+        }
+
         $parameters[] = $parameter->getConcreteString();
       }