[flang] Fixed unittest build issue with older gcc.

[vzakhari]

Summary

I am getting this error with check-flang:

ld.lld: error: undefined symbol: mlir::SuccessorRange::SuccessorRange(mlir::Operation*)
>>> referenced by Operation.h:549 (/llvm-project/llvm/../mlir/include/mlir/IR/Operation.h:549)
>>>               CMakeFiles/FlangRuntimeTests.dir/Allocatable.cpp.o:(mlir::Operation::getSuccessors())

The buildbots are okay, so I guess it has something to do with
gcc-9 that I am using.

[vzakhari]

It fails in other reviews like this: https://buildkite.com/llvm-project/premerge-checks/builds/131469#0185c7d2-1cf4-4b7e-a03d-61e2f2e0b92d

David, I am going to merge it without approval.

[DavidTruby]

Thanks for fixing this!

flang/unittests/Runtime/Allocatable.cpp

 //===-- flang/unittests/Runtime/Allocatable.cpp--------- ---------*- C++-*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #include "flang/Runtime/allocatable.h"
 #include "gtest/gtest.h"
 #include "tools.h"
-#include "flang/Common/Fortran.h"
-#include "flang/ISO_Fortran_binding.h"
-#include "flang/Optimizer/Builder/FIRBuilder.h"
-#include "flang/Optimizer/Support/InitFIR.h"
-#include "flang/Optimizer/Support/KindMapping.h"
-#include "flang/Runtime/descriptor.h"
-#include "flang/Runtime/memory.h"
 
 using namespace Fortran::runtime;
 
 static OwningPtr<Descriptor> createAllocatable(
     Fortran::common::TypeCategory tc, int kind, int rank = 1) {
   return Descriptor::Create(TypeCode{tc, kind}, kind, nullptr, rank, nullptr,
       CFI_attribute_allocatable);
 }