Drop trivial special members

Do not define trivial constructors or destructors unless necessary
(e.g., for implementing pimpl) following the rule of zero. Define
virtual base class destructors out-of-line to avoid emitting vtables in
every translation unit.

Change-Id: Iea2d8978e62a8ee5a97b86cbb4e858d56e0fb274

internal/ceres/gradient_checking_cost_function_test.cc

@@ -268,7 +268,6 @@ class UnaryCostFunction : public CostFunction {
     set_num_residuals(num_residuals);
     mutable_parameter_block_sizes()->push_back(parameter_block_size);
   }
-  ~UnaryCostFunction() override = default;
 
   bool Evaluate(double const* const* parameters,
                 double* residuals,