[flang][runtime] Accommodate change of type in assignment to allocatable (#141988)

When an assignment to a derived type allocatable requires
(re)allocation, its type may change to that of the right-hand side. The
code didn't update its derived type pointer, leading to the wrong type
being put into the descriptors created for elemental defined assignment
subroutine calls.

Fixes https://github.com/llvm/llvm-project/issues/141835.

flang-rt/lib/runtime/assign.cpp

@@ -330,6 +330,7 @@ RT_API_ATTRS void Assign(Descriptor &to, const Descriptor &from,
       }
       flags &= ~NeedFinalization;
       toElementBytes = to.ElementBytes(); // may have changed
+      toDerived = toAddendum ? toAddendum->derivedType() : nullptr;
     }
   }
   if (toDerived && (flags & CanBeDefinedAssignment)) {