[MLIR][Presburger] Move functionality from IntegerPolyhedron to IntegerRelation

This patch moves all functionality from IntegerPolyhedron to IntegerRelation.
IntegerPolyhedron is now implemented as a relation with no domain. All existing
functionality is extended to work on relations.

This patch does not affect external users like FlatAffineConstraints as they
can still continue to use IntegerPolyhedron abstraction.

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

Reviewed By: arjunp

Differential Revision: https://reviews.llvm.org/D120652