Implement tracking of BUILD.gn files used to define target, toolchain or

tools/gn/loader.cc

Index: tools/gn/loader.cc
diff --git a/tools/gn/loader.cc b/tools/gn/loader.cc
index 0ede3c95548389882fab5481a9f3ad532cb5e061..417a8914d17e01b492d51b28847356ffcd59dea5 100644
--- a/tools/gn/loader.cc
+++ b/tools/gn/loader.cc
@@ -1,7 +1,6 @@
 // Copyright (c) 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
-
 #include "tools/gn/loader.h"
 
 #include "base/bind.h"
@@ -251,6 +250,11 @@ void LoaderImpl::BackgroundLoadFile(const Settings* settings,
   }
 
   Scope our_scope(settings->base_config());
+
+  const auto location = root->GetRange().begin();
+  if (!location.is_null())
+    our_scope.AddInputFile(location.file());
+
   ScopePerFileProvider per_file_provider(&our_scope, true);
   our_scope.set_source_dir(file_name.GetDir());