[flang][OpenMP][NFC] Further refactoring for genOpWithBody &

`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.

Author: ergawy