[MLIR][Affine] Allow `<=` in IntegerSet constraints

This patch extends the affine parser to allow affine constraints with `<=`.
This is useful in writing unittests for Presburger library and test in general.

The internal storage and printing of IntegerSet is still in the original format.

Reviewed By: bondhugula

Differential Revision: https://reviews.llvm.org/D129046