message: Fix compiler warning introduced by r358261
  Add a virtual destructor to the class with virtual methods.
diff: @@ -160,6 +160,8 @@ private:
  template <typename ResultT = void>
  class FPOProgramASTVisitor {
  protected:
  + virtual ~FPOProgramASTVisitor() = default;
  +
  virtual ResultT Visit(FPOProgramNodeBinaryOp &binary,
  FPOProgramNode *&ref) = 0;
  virtual ResultT Visit(FPOProgramNodeUnaryOp &unary, FPOProgramNode *&ref) = 0;
  