[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.

Author: DavidSpickett

flang/include/flang/Runtime/descriptor.h

@@ -456,6 +456,7 @@ class alignas(Descriptor) StaticDescriptor {
     assert(descriptor().rank() <= maxRank);
     assert(descriptor().SizeInBytes() <= byteSize);
     if (DescriptorAddendum * addendum{descriptor().Addendum()}) {
+      (void)addendum;
       assert(hasAddendum);
       assert(addendum->LenParameters() <= maxLengthTypeParameters);
     } else {