[MLIR] Introduce LexSimplex to support lexicographic optimization

This patch introduces a class LexSimplex that can currently be used to find the
lexicographically minimal rational point in an IntegerPolyhedron. This is a
series of patches leading to computing the lexicographically minimal integer
lattice point as well parametric lexicographic minimization.

Reviewed By: Groverkss

Differential Revision: https://reviews.llvm.org/D117437