[MLIR][Presburger] Add support for PresburgerRelation

This patch adds supports for union of relations (PresburgerRelation).  Along
with this, support for PresburgerSet is also maintained.

This patch is part of a series of patches to add support for relations in
Presburger library.

Reviewed By: arjunp

Differential Revision: https://reviews.llvm.org/D121417