Allow creation and modification of scopes in GN.

tools/gn/function_foreach.cc

 // Copyright (c) 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "tools/gn/err.h"
 #include "tools/gn/functions.h"
 #include "tools/gn/parse_node_value_adapter.h"
 #include "tools/gn/parse_tree.h"
 #include "tools/gn/scope.h"
 
@@ skipping 77 matching lines @@
     scope->SetValue(loop_var, cur, function);
     block->Execute(scope, err);
     if (err->has_error())
       return Value();
   }
 
   // Put back loop var.
   if (old_loop_value_ptr) {
     // Put back old value. Use the copy we made, rather than use the pointer,
     // which will probably point to the new value now in the scope.
-    scope->SetValue(loop_var, old_loop_value, old_loop_value.origin());
+    scope->SetValue(loop_var, std::move(old_loop_value),
+                    old_loop_value.origin());
   } else {
     // Loop variable was undefined before loop, delete it.
     scope->RemoveIdentifier(loop_var);
   }
 
   return Value();
 }
 
 }  // namespace functions