[MLIR][Presburger][NFC] Cleanup PresburgerSet

This patch cleans up the interface to PresburgerSet. At a high level it does
the following changes:

  - Move member functions around to have constructors at top and print/dump
    at end.
  - Move a private function to be a static function instead.
  - Change member functions of type "getAllIntegerPolyhedron" to "getAllPolys"
    instead.
  - Improve documentation for PresburgerSet.

Reviewed By: arjunp

Differential Revision: https://reviews.llvm.org/D121027