[flang] Silence warning in release builds

/home/david.spickett/llvm-project/flang/include/flang/Runtime/descriptor.h:458:30: warning: variable 'addendum' set but not used [-Wunused-but-set-variable]
  458 |     if (DescriptorAddendum * addendum{descriptor().Addendum()}) {
      |

The if body is purely asserts.