[flang] Adapt PointerIsAssociatedWith for empty derived-type

[clementval]

Summary

When a derived-type as no component, its elem_len will be set to
zero when emboxed. Update the function to let empty derived-type
pointer/target succeed the test.

Example extracted from gfortran test pointer_init_8

module m
  type :: c
  end type c
  type, extends(c) :: d
  end type d
  type(c), target :: x
end module

use m
  class(c), pointer :: px => x

  if (.not. associated(px, x))   STOP 1
end

flang/runtime/pointer.cpp

bool RTNAME(PointerIsAssociated)(const Descriptor &pointer) {
   return pointer.raw().base_addr != nullptr;
 }
 
 bool RTNAME(PointerIsAssociatedWith)(
     const Descriptor &pointer, const Descriptor *target) {
   if (!target) {
     return pointer.raw().base_addr != nullptr;
   }
-  if (!target->raw().base_addr || target->ElementBytes() == 0) {
+  if (!target->raw().base_addr) {
     return false;
   }
   int rank{pointer.rank()};
   if (pointer.raw().base_addr != target->raw().base_addr ||
       pointer.ElementBytes() != target->ElementBytes() ||
       rank != target->rank()) {
     return false;
   }