[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

Author: Groverkss

mlir/include/mlir/Analysis/Presburger/PWMAFunction.h

@@ -139,7 +139,7 @@ class MultiAffineFunction : protected IntegerPolyhedron {
 ///
 /// Support is provided to compare equality of two such functions as well as
 /// finding the value of the function at a point.
-class PWMAFunction : public PresburgerSpace {
+class PWMAFunction : private PresburgerSpace {
 public:
   PWMAFunction(const PresburgerSpace &space, unsigned numOutputs)
       : PresburgerSpace(space), numOutputs(numOutputs) {