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

tools/gn/import_manager.cc

Index: tools/gn/import_manager.cc
diff --git a/tools/gn/import_manager.cc b/tools/gn/import_manager.cc
index c9a44d4eaf4d69f52ed2c367fab92b0d4974e0dc..67c992dd422f46cf623e315576ec9340a5f4e78d 100644
--- a/tools/gn/import_manager.cc
+++ b/tools/gn/import_manager.cc
@@ -27,6 +27,10 @@ std::unique_ptr<Scope> UncachedImport(const Settings* settings,
   std::unique_ptr<Scope> scope(new Scope(settings->base_config()));
   scope->set_source_dir(file.GetDir());
 
+  const Location& location = node->GetRange().begin();
+  if (!location.is_null())
+    scope->AddInputFile(location.file());
+
   // Don't allow ScopePerFileProvider to provide target-related variables.
   // These will be relative to the imported file, which is probably not what
   // people mean when they use these.