Improve ArcanistXHPASTLinter::LINT_TOSTRING_EXCEPTION

Summary:
The `ArcanistXHPASTLinter::LINT_TOSTRING_EXCEPTION` linter rule was introduced in D12854, but fails for the following:

  - Interfaces which declare the `__toString()` method.
  - Classes which mark the `__toString()` method as `abstract`.

Test Plan: Added test cases.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D12892

src/lint/linter/ArcanistXHPASTLinter.php

@@ -4144,9 +4144,13 @@ final class ArcanistXHPASTLinter extends ArcanistBaseXHPASTLinter {
         continue;
       }
 
-      $throws = $method
-        ->getChildOfType(5, 'n_STATEMENT_LIST')
-        ->selectDescendantsOfType('n_THROW');
+      $statements = $method->getChildByIndex(5);
+
+      if ($statements->getTypeName() != 'n_STATEMENT_LIST') {
+        continue;
+      }
+
+      $throws = $statements->selectDescendantsOfType('n_THROW');
 
       foreach ($throws as $throw) {
         $this->raiseLintAtNode(

src/lint/linter/__tests__/xhpast/tostring-exception.lint-test

@@ -14,5 +14,13 @@ class MyOtherClass {
     return 'Success';
   }
 }
+
+interface SomeInterface {
+  public function __toString();
+}
+
+abstract class SomeAbstractClass {
+  abstract public function __toString();
+}
 ~~~~~~~~~~
 error:5:7