message: Fix printing of TriangularSolve HLO
  Previously it was calling AplhaNum on a bool what implicitly converted it to
  an int what can't be parsed as a bool attribute by the hlo parser.
diff: @@ -218,9 +218,12 @@ HloInstructionProto HloTriangularSolveInstruction::ToProto() const {
  std::vector<string> HloTriangularSolveInstruction::ExtraAttributesToStringImpl(
  const HloPrintOptions& options) const {
  - return {StrCat("left_side=", triangular_solve_options_.left_side()),
  - StrCat("lower=", triangular_solve_options_.lower()),
  - StrCat("unit_diagonal=", triangular_solve_options_.unit_diagonal()),
  + return {
  + StrCat("left_side=",
  + triangular_solve_options_.left_side() ? "true" : "false"),
  + StrCat("lower=", triangular_solve_options_.lower() ? "true" : "false"),
  + StrCat("unit_diagonal=",
  + triangular_solve_options_.unit_diagonal() ? "true" : "false"),
  StrCat("transpose_a=", TriangularSolveOptions_Transpose_Name(
  triangular_solve_options_.transpose_a()))};
  }
  