[gn build] (manually) port aee6c86c4d better

"EmptyNodeIntrospection.inc.in" needs to be a source of the action,
so that ninja knows to rerun this action if that input changes.

utils/gn/secondary/clang/lib/Tooling/BUILD.gn

@@ -3,10 +3,10 @@
 action("node_introspection_inc") {
   script = "DumpTool/generate_cxx_src_locs.py"
   outputs = [ "$target_gen_dir/clang/Tooling/NodeIntrospection.inc" ]
+  sources = [ "EmptyNodeIntrospection.inc.in" ]
   args = [
     "--use-empty-implementation=1",
-    "--empty-implementation=" +
-        rebase_path("EmptyNodeIntrospection.inc.in", root_build_dir),
+    "--empty-implementation=" + rebase_path(sources[0], root_build_dir),
     "--output-file=" + rebase_path(outputs[0], root_build_dir),
   ]
 }