[Offload][OpenMP] Support shadow-pointer tracking for Fortran descriptors. (#158370)

This change adds support for saving full contents of attached Fortran
descriptors, and not just their pointee address, in the shadow-pointer
table.

With this, we now support:
* comparing full contents of descriptors to check whether a previous
shadow-pointer entry is stale;
* restoring the full contents of descriptors

And with that, we can now use ATTACH map-types (added in #149036) for
mapping Fortran pointer/allocatable arrays, and array-sections on them.
e.g.:

```f90
  integer, allocatable :: x(:)
  !$omp target enter data map(to: x(:))
```

as:

```
  void* addr_of_pointee = allocated(x) ? &x(1) : nullptr;
  int64_t sizeof_pointee = allocated(x) ? sizeof(x(:)) : 0

  addr_of_pointee,    addr_of_pointee, sizeof_pointee,     TO
  addr_of_descriptor, addr_of_pointee, size_of_descriptor, ATTACH
```