[Refactor Xcode Objects] Decouple file references and indexing target.

tools/gn/xcode_object.h

Index: tools/gn/xcode_object.h
diff --git a/tools/gn/xcode_object.h b/tools/gn/xcode_object.h
index 83a776ac405ab99b9f6a427c5f2eb57c32dbf08d..f56de09a7806658b2cf4c870be1fddc332e6bdd5 100644
--- a/tools/gn/xcode_object.h
+++ b/tools/gn/xcode_object.h
@@ -275,10 +275,15 @@ class PBXProject : public PBXObject {
              const PBXAttributes& attributes);
   ~PBXProject() override;
 
+  void AddSourceFileToIndexingTarget(const std::string& navigator_path,
+                                     const std::string& source_path);
   void AddSourceFile(const std::string& navigator_path,
-                     const std::string& source_path);
+                     const std::string& source_path,
+                     PBXNativeTarget* target);
+
   void AddAggregateTarget(const std::string& name,
                           const std::string& shell_script);
+  void AddIndexingTarget();
   void AddNativeTarget(const std::string& name,
                        const std::string& type,
                        const std::string& output_name,