[flang][runtime] Fixed memory leak in Assign().

[vzakhari]

Summary

The temporary descriptor must be either Pointer or Allocatable,
otherwise its memory will not be freed.

flang/runtime/assign.cpp

if (mustDeallocateLHS) {
       to.set_base_addr(nullptr);
     } else if (!isSimpleMemmove()) {
       // Handle LHS/RHS aliasing by copying RHS into a temp, then
       // recursively assigning from that temp.
       auto descBytes{from.SizeInBytes()};
       StaticDescriptor<maxRank, true, 16> staticDesc;
       Descriptor &newFrom{staticDesc.descriptor()};
       std::memcpy(&newFrom, &from, descBytes);
+      // Pretend the temporary descriptor is for an ALLOCATABLE
+      // entity, otherwise, the Deallocate() below will not
+      // free the descriptor memory.
+      newFrom.raw().attribute = CFI_attribute_allocatable;
       auto stat{ReturnError(terminator, newFrom.Allocate())};
       if (stat == StatOk) {
         char *toAt{newFrom.OffsetElement()};
         std::size_t fromElements{from.Elements()};
         if (from.IsContiguous()) {
           std::memcpy(
               toAt, from.OffsetElement(), fromElements * fromElementBytes);
         } else {