message: Save a map lookup in the eager attribute builder
  The temporary was causing destruction of an arena-allocated protobuf message, which apparently isn't thread safe. I don't know if I'll be able to get rid of all similar cases, but this at least looks like a good change.
diff: @@ -163,10 +163,7 @@ class AttrBuilder {
  DCHECK(!node_def_finalized_)
  << "Calling SetInAttrValueMap after BuildNodeDef.";
  // If attribute is set more than once, its first value prevails
  - if (AttrSlice(m).Find(attr_name) == nullptr) {
  - SetAttrValue(value, &attr_tmp_);
  - m->insert(AttrValueMap::value_type(attr_name, attr_tmp_));
  - }
  + m->insert({attr_name, value});
  }
  void AddAttrIfNotPresent(StringPiece attr_name, const AttrValue& value);
  