[flang] add fir.box_offset operation (#73641)

This operation allows computing the address of descriptor fields. It is
needed to help attaching descriptors in OpenMP/OpenACC target region.
The pointers inside the descriptor structure must be mapped too, but the
fir.box is abstract, so these fields cannot be computed with
fir.coordinate_of.

To preserve the abstraction of the descriptor layout in FIR, introduce
an operation specifically to !fir.ref<fir.box<>> address fields based on
field names (base_addr or derived_type).