[flang][acc] Implement MappableType's generatePrivateInit (#148302)
The recipe body generation was moved from lowering into FIR's
implementation of MappableType API. And now since all Fortran variable
types implement this type, lowering of OpenACC was updated to use this
API directly. No test changes were needed - all of the private,
firstprivate, and recipe tests get the same body as before.