[flang] Fix CFI_CDESC_T(rank) for C (#66260)

The CFI_CDESC_T(rank) macro defined for C (not C++) in
ISO_Fortran_binding.h incorporates a cdesc_t structure as a member,
which works for data layout but doesn't allow for direct access to its
members. (The C++ definition can use inheritance.)

Restructure the definitions in that header file so that
CFI_CDESC_T(rank) for C defines a struct with the expected members.