[flang] Fixed build issue after 88f07a73

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)