message: Add user provided constructor to XlaClusterInfo - the default one is deleted
  with libstdc++ >= 7.3, as there std::map does not have a user provided default
  constructor, and XlaClusterInfo contains it as a const member. This is diagnosed
  by clang.
diff: @@ -117,6 +117,22 @@ Status PreprocessForEncapsulation(Graph* g,
  // Information for XLA computation.
  struct XlaClusterInfo {
  + // The implicit default constructor is deleted because host_compute_core is a
  + // const member whose type (std::map) doesn't necessarily have a user provided
  + // constructor - while libc++ and libstdc++ 4.8 provide a user defined
  + // default constructor, libstdc++ at least >= 7.3 does not.
  + // See also c++11 [class.ctor] p5.
  + // TODO(klimek): In c++17 we'll be able to initialize host_compute_core
  + // without losing aggregate initialization, which allows us to get rid of
  + // the constructor definitions again.
  + XlaClusterInfo() {}
  + XlaClusterInfo(const string& cluster_name,
  + const NameAttrList& func_name_attrs, Node* node,
  + const std::map<string, int>& host_compute_core)
  + : cluster_name(cluster_name),
  + func_name_attrs(func_name_attrs),
  + node(node),
  + host_compute_core(host_compute_core) {}
  // XLA cluster name. It might be different from `func_name`.
  const string cluster_name;
  // Name and attributes of XLA computation function.
  