[MLIR][Presburger] Make PWMAFunction inheritence from space private

This patch makes inheritence from PresburgerSpace for PWMAFunction private.

The reasoning for this patch is to prevent implicit conversion to
PresburgerSpace from PWMAFunction and to not expose all functions exposed by
PresburgerSpace in PWMAFunction.

Reviewed By: arjunp

Differential Revision: https://reviews.llvm.org/D123076