[flang][OpenMP][NFC] Further refactoring for `genOpWithBody` & (#80839)

`createBodyOfOp`

This refactors the arguments to the above functions in 2 ways:
- Combines the 2 structs of arguments into one since they were almost
identical.
- Replaces the `args` argument with a callback to a rebion-body
generation function. This is a preparation for delayed privatization as
we will need different callbacks for ws loops and parallel ops with
delayed privatization.