[flang][runtime] Accommodate change of type in assignment to allocatable (llvm#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 llvm#141835.

Author: klausler

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)) {