Add the class MultiAffineFunction which represents functions whose domain is an
IntegerPolyhedron and which produce an output given by a tuple of affine
expressions in the IntegerPolyhedron's ids.

Also add support for piece-wise MultiAffineFunctions, which are defined on a
union of IntegerPolyhedrons, and may have different output affine expressions
on each IntegerPolyhedron. Thus the function is affine on each individual
IntegerPolyhedron piece in the domain.

This is part of a series of patches leading up to parametric integer programming.

Depends on D118778.

Reviewed By: Groverkss

Differential Revision: https://reviews.llvm.org/D118779