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

tools/gn/function_toolchain.cc

Index: tools/gn/function_toolchain.cc
diff --git a/tools/gn/function_toolchain.cc b/tools/gn/function_toolchain.cc
index baac8ee7dfd57b995efad544eda5b5c88c2eb643..adbab300a9270752354f81cfccda2dd8489cbed2 100644
--- a/tools/gn/function_toolchain.cc
+++ b/tools/gn/function_toolchain.cc
@@ -453,7 +453,8 @@ Value RunToolchain(Scope* scope,
 
   // This object will actually be copied into the one owned by the toolchain
   // manager, but that has to be done in the lock.
-  std::unique_ptr<Toolchain> toolchain(new Toolchain(scope->settings(), label));
+  std::unique_ptr<Toolchain> toolchain(
+      new Toolchain(scope->settings(), label, scope->input_files()));
   toolchain->set_defined_from(function);
   toolchain->visibility().SetPublic();