message: Make comparison functors const
  In libc++ std::map and std::multimap call the comparison functor
  from a const object, which requires the `operator()` to be a const
  method.
diff: @@ -174,7 +174,9 @@ bool HasResourceInputOrOutput(const Node& node) {
  }
  struct NodeCompare {
  - bool operator()(const Node* a, const Node* b) { return a->id() < b->id(); }
  + bool operator()(const Node* a, const Node* b) const {
  + return a->id() < b->id();
  + }
  };
  using OrderedNodeSet = std::set<Node*, NodeCompare>;
  