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

tools/gn/settings.cc

Index: tools/gn/settings.cc
diff --git a/tools/gn/settings.cc b/tools/gn/settings.cc
index 034e601a3476b742b98be657d69e72917e2d3cec..2efcccdfedf9735e3624321f663a1bed9f487bfc 100644
--- a/tools/gn/settings.cc
+++ b/tools/gn/settings.cc
@@ -12,8 +12,9 @@ Settings::Settings(const BuildSettings* build_settings,
                    const std::string& output_subdir_name)
     : build_settings_(build_settings),
       import_manager_(),
-      base_config_(this),
+      base_config_(this, {}),
       greedy_target_generation_(false) {
+  base_config_.AddSourceFile(build_settings->build_config_file());
   if (output_subdir_name.empty()) {
     toolchain_output_dir_ = build_settings->build_dir();
   } else {