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

[Groverkss]

Summary

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.

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

 /// this class is undefined. The domains need not cover all possible points;
 /// this represents a partial function and so could be undefined at some points.
 ///
 /// As in PresburgerSets, the input ids are partitioned into dimension ids and
 /// symbolic ids.
 ///
 /// 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) {
     assert(getNumDomainIds() == 0 && "Set type space should zero domain ids.");
     assert(getNumLocalIds() == 0 && "PWMAFunction cannot have local ids.");
     assert(numOutputs >= 1 && "The function must output something!");
   }