[MLIR] Turns swapId into a FlatAffineConstraints member func

`swapId` used to be a static function in `AffineStructures.cpp`. This diff makes it accessible from the external world by turning it into a member function of `FlatAffineConstraints`. This will be very helpful for other projects that need to manipulate the content of `FlatAffineConstraints`.

Differential Revision: https://reviews.llvm.org/D87766