[flang] Set the addendum when establishing pointer section in descriptor

If the source has an addendum, the descriptor that is being established
to describe a section over the source needs to copy the addendum so that
derived type information is correctly set in the descriptor being
established.

This allows namelist IO with derived type to work correctly.

Differential Revision: https://reviews.llvm.org/D113258

flang/runtime/descriptor.cpp

@@ -241,6 +241,13 @@ bool Descriptor::EstablishPointerSection(const Descriptor &source,
     }
   }
   raw_.rank = newRank;
+  if (const auto *sourceAddendum = source.Addendum()) {
+    if (auto *addendum{Addendum()}) {
+      *addendum = *sourceAddendum;
+    } else {
+      return false;
+    }
+  }
   return CFI_section(&raw_, &source.raw_, lower, upper, stride) == CFI_SUCCESS;
 }