Clang: Fix possible future watcher issues in local renaming

Use the same code pattern as in asynchronous branch of
CppUseSelectionsUpdater::update.

Change-Id: I7c06b883a5abe1324cb63fa410247ef7f50321ae
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@qt.io>

src/plugins/clangcodemodel/clangrefactoringengine.h

@@ -55,7 +55,7 @@ public:
 
 private:
     using FutureCursorWatcher = QFutureWatcher<CppTools::CursorInfo>;
-    FutureCursorWatcher m_watcher;
+    std::unique_ptr<FutureCursorWatcher> m_watcher;
 };
 
 } // namespace ClangRefactoring