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

tools/gn/setup.h

Index: tools/gn/setup.h
diff --git a/tools/gn/setup.h b/tools/gn/setup.h
index 68a8f0b35e53501e004097d822899136ae4c366a..af067837680c9eed39d8e58949b0f3eae6313439 100644
--- a/tools/gn/setup.h
+++ b/tools/gn/setup.h
@@ -85,6 +85,10 @@ class Setup {
   Builder& builder() { return builder_; }
   LoaderImpl* loader() { return loader_.get(); }
 
+  const InputFile* dotfile_input_file() const {
+    return dotfile_input_file_.get();
+  }
+
   // Name of the file in the root build directory that contains the build
   // arguements.
   static const char kBuildArgFileName[];