[MLIR][Presburger] Add support for IntegerRelation

This patch adds a class to represent a relation in Presburger Library.

This patch only adds the skeleton class. Functionality from IntegerPolyhedron
will be moved to IntegerRelation in later patches to make it easier to review.

This patch is a part of a series of patches adding support for relations in
Presburger Library.

Reviewed By: arjunp

Differential Revision: https://reviews.llvm.org/D120156