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 05c764bfbdf65ab5524c8c15ae48221b7a2d63dc..842a4e7b7ba3bf96aae29c6fcf6d046b793366fb 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->source_files_hashes()));
   toolchain->set_defined_from(function);
   toolchain->visibility().SetPublic();