[Flang][OpenMP][Lower] NFC: Move clause processing helpers into the ClauseProcessor (#85258)

This patch moves some code in PFT to MLIR OpenMP lowering to the
`ClauseProcessor` class. This is so that some behavior that is related
to certain clauses stays within the `ClauseProcessor` and it's not the
caller the one responsible for always doing this when the clause is
present.