[MLIR][Presburger] Remove inheritance from PresburgerSpace in IntegerRelation, Presbu...

[MLIR][Presburger] Remove inheritance from PresburgerSpace in IntegerRelation, PresburgerRelation and PWMAFunction

This patch removes inheritence from PresburgerSpace in IntegerRelation and
instead makes it a member of these classes.

This is required for three reasons:
  - It prevents implicit casting to PresburgerSpace.
  - Not all functions of PresburgerSpace need to be exposed by the deriving classes.
  - IntegerRelation and IntegerPolyhedron are defined in a PresburgerSpace. It
    makes more sense for the space to be a member instead of them inheriting from
    a space.

Reviewed By: arjunp, ftynse

Differential Revision: https://reviews.llvm.org/D123585