[flang][OpenMP][NFC] Outline `genOpWithBody` & `createBodyOfOp` args (#80817)

This PR outlines the arguments of the open CodeGen functions into 2
separate structs. This was, in part, motivated by the delayed
privatization WIP #79862 where we had to extend the signatures of both
functions containing quite a bit of default values (`nullptr`, `false`).
This PR does not add any new arguments yet though, just outlines the
existing ones.