[flang] Fixed unittest build issue with older gcc.

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.

Differential Revision: https://reviews.llvm.org/D142069