message: Remove unnecessary heterogenous lookup ifdef.
  We're using C++17 throughout, so these code-cluttering
  ifdefs are not needed anymore.
diff: @@ -72,29 +72,15 @@ class BijectiveMap {
  // Lookup value given key. Returns nullptr if not found.
  template <typename ConvertibleToKey>
  const V* find_forward(const ConvertibleToKey& k) const {
  - auto iter = forward_map.find(
  -#if __cplusplus >= 201402L
  - k // heterogeneous lookup enabled (C++14)
  -#else
  - // copy temporary rvalue if necessary
  - ForwardReferenceElseConstruct<K>()(k)
  -#endif
  - );
  - return iter == forward_map.end() ? nullptr : iter->second;
  + const auto found = forward_map.find(k);
  + return found == forward_map.end() ? nullptr : found->second;
  }
  // Lookup key given value. Returns nullptr if not found.
  template <typename ConvertibleToValue>
  const K* find_reverse(const ConvertibleToValue& v) const {
  - auto iter = reverse_map.find(
  -#if __cplusplus >= 201402L
  - v // heterogeneous lookup enabled (C++14)
  -#else
  - // copy temporary rvalue if necessary
  - ForwardReferenceElseConstruct<V>()(v)
  -#endif
  - );
  - return iter == reverse_map.end() ? nullptr : iter->second;
  + const auto found = reverse_map.find(v);
  + return found == reverse_map.end() ? nullptr : found->second;
  }
  // Returns true if successfully inserted new pair.
  