performance-use-std-move

Suggests insertion of std::move(...) to turn copy assignment operator calls into move assignment ones, when deemed valid and profitable.

void assign(std::vector<int>& out) {
  std::vector<int> some = make_vector();
  use_vector(some);
  out = some;
}

// becomes

void assign(std::vector<int>& out) {
  std::vector<int> some = make_vector();
  use_vector(some);
  out = std::move(some);
}