Function cloneUnique

ThirdParty/jsoncpp/vtkjsoncpp/jsoncpp.cpp:2490–2496

namespace Json {
template <typename T>
static std::unique_ptr<T> cloneUnique(const std::unique_ptr<T>& p) {
  std::unique_ptr<T> r;
  if (p) {
    r = std::unique_ptr<T>(new T(*p));
  }
  return r;
}

// This is a walkaround to avoid the static initialization of Value::null.
// kNull must be word-aligned to avoid crashing on ARM.  We use an alignment of