This simplifies the usage of `__less` by making the class not depend on the types compared, but instead the `operator()`. We can't remove the template completely because we explicitly instantiate `std::__sort` with `__less<T>`.
Reviewed By: ldionne, #libc
Spies: arichardson, EricWF, libcxx-commits, mgrang
Differential Revision: https://reviews.llvm.org/D145285