[flang][NFC] Enable and migrate to new fold API

See https://discourse.llvm.org/t/psa-new-improved-fold-method-signature-has-landed-please-update-your-downstream-projects/67618 for context

This simply ports all dialects in flang to use the new fold API. These were relatively little and basically just a function signature change, since in-tree folds did not make use of any of the constant operands values.

Differential Revision: https://reviews.llvm.org/D141488