message: Change string to std::string.
diff: namespace mlir {
  // Simple helper function that returns a string as printed from a op.
  -template <typename T>
  -static string debugString(T &op) {
  +template <typename T> static std::string debugString(T &op) {
  std::string instr_str;
  llvm::raw_string_ostream os(instr_str);
  op.print(os);
  