[MLIR] Move LinearTransform to Presburger/

This patch moves LinearTransform to Presburger/ and makes it use
IntegerPolyhedron instead of FlatAffineConstraints. Also modifies its usage in
`FlatAffineConstraints::findIntegerSample` to support the changes.

This patch is part of a series of patches for moving presburger math functionality into Presburger directory.

Reviewed By: arjunp

Differential Revision: https://reviews.llvm.org/D116311