[flang] Fixed build issue after 88f07a736bbc3f0062d7d8f4032f0b54aff5c018

[vzakhari]

Summary

nullptr matches both against ::mlir::UnitAttr and ::mlir::TypeRange,
so the following two candidates fit:
static void mlir::omp::OrderedRegionOp::build(::mlir::OpBuilder &odsBuilder,

::mlir::OperationState &odsState,
/*optional*/::mlir::UnitAttr simd)

static void mlir::omp::OrderedRegionOp::build(::mlir::OpBuilder &odsBuilder,

::mlir::OperationState &odsState,
::mlir::TypeRange resultTypes,
/*optional*/bool simd = false)

flang/lib/Lower/OpenMP.cpp

if (blockDirective.v == llvm::omp::OMPD_parallel) {
     createBodyOfOp<omp::MasterOp>(masterOp, converter, currentLocation, eval);
   } else if (blockDirective.v == llvm::omp::OMPD_single) {
     auto singleOp = firOpBuilder.create<mlir::omp::SingleOp>(
         currentLocation, allocateOperands, allocatorOperands, nowaitAttr);
     createBodyOfOp<omp::SingleOp>(singleOp, converter, currentLocation, eval,
                                   &opClauseList);
   } else if (blockDirective.v == llvm::omp::OMPD_ordered) {
     auto orderedOp = firOpBuilder.create<mlir::omp::OrderedRegionOp>(
-        currentLocation, /*simd=*/nullptr);
+        currentLocation, /*simd=*/false);
     createBodyOfOp<omp::OrderedRegionOp>(orderedOp, converter, currentLocation,
                                          eval);
   } else if (blockDirective.v == llvm::omp::OMPD_task) {
     auto taskOp = firOpBuilder.create<mlir::omp::TaskOp>(
         currentLocation, ifClauseOperand, finalClauseOperand, untiedAttr,
         mergeableAttr, /*in_reduction_vars=*/ValueRange(),
         /*in_reductions=*/nullptr, priorityClauseOperand, allocateOperands,
         allocatorOperands);