[MLIR][Presburger] Make the SimplexBase constructor protected

[arjunp]

Summary

This is not supposed to be instantiated directly anyway.

[Groverkss]

LGTM.

mlir/include/mlir/Analysis/Presburger/Simplex.h

 /// if this functionality is used, then on rolling back the exact basis will
 /// also be restored. This is used by LexSimplex because its algorithm, unlike
 /// Simplex, is sensitive to the exact basis used at a point.
 class SimplexBase {
 public:
   SimplexBase() = delete;
   virtual ~SimplexBase() = default;
 
-  /// Construct a SimplexBase with the specified number of variables and fixed
-  /// columns.
-  ///
-  /// For example, Simplex uses two fixed columns: the denominator and the
-  /// constant term, whereas LexSimplex has an extra fixed column for the
-  /// so-called big M parameter. For more information see the documentation for
-  /// LexSimplex.
-  SimplexBase(unsigned nVar, bool mustUseBigM);
-
   /// Returns true if the tableau is empty (has conflicting constraints),
   /// false otherwise.
   bool isEmpty() const;
 
   /// Add an inequality to the tableau. If coeffs is c_0, c_1, ... c_n, where n
   /// is the current number of variables, then the corresponding inequality is
   /// c_n + c_0*x_0 + c_1*x_1 + ... + c_{n-1}*x_{n-1} >= 0.
   virtual void addInequality(ArrayRef<int64_t> coeffs) = 0;
public:
   /// Add all the constraints from the given IntegerRelation.
   void intersectIntegerRelation(const IntegerRelation &rel);
 
   /// Print the tableau's internal state.
   void print(raw_ostream &os) const;
   void dump() const;
 
 protected:
+  /// Construct a SimplexBase with the specified number of variables and fixed
+  /// columns.
+  ///
+  /// For example, Simplex uses two fixed columns: the denominator and the
+  /// constant term, whereas LexSimplex has an extra fixed column for the
+  /// so-called big M parameter. For more information see the documentation for
+  /// LexSimplex.
+  SimplexBase(unsigned nVar, bool mustUseBigM);
+
   enum class Orientation { Row, Column };
 
   /// An Unknown is either a variable or a constraint. It is always associated
   /// with either a row or column. Whether it's a row or a column is specified
   /// by the orientation and pos identifies the specific row or column it is
   /// associated with. If the unknown is restricted, then it has a
   /// non-negativity constraint associated with it, i.e., its sample value must
   /// always be non-negative and if it cannot be made non-negative without