[mlir] Partially revert removal of old `fold` method

Mehdi noted in https://reviews.llvm.org/D144391 that given the low cost of keeping the old `fold` method signature working and the difficulty of writing a `FoldAdaptor` oneself, it'd be nice to keep the support for the sake of Ops written manually in C++.
This patch therefore partially reverts the removal of the old `fold` method by still allowing the old signature to be used. The active use of it is still discouraged and ODS will always generate the new method using `FoldAdaptor`s.

I'd also like to note that the previous ought to have broken some manually defined `fold` methods in-tree that are defined here: https://github.com/llvm/llvm-project/blob/23bcd6b86271f1c219a69183a5d90654faca64b8/mlir/include/mlir/Dialect/Affine/IR/AffineOps.h#L245 It seems like these are not part of the regressions tests however...

Differential Revision: https://reviews.llvm.org/D144591