Disallow wrapping input iterators.

Iterator::Next() has this line:

  if (it_ != end(*self_)) return &*it_++;

The reference returned by *it_ doesn't have to be valid after the iterator is
incremented; input iterators might create these values on the fly (and don't
store them).

TGP: https://test.corp.google.com/ui#id=OCL:387627910:BASE:391752438:1629382590029:3d364c60

See also: http://g/c-users/xvDy7spjpeM

PiperOrigin-RevId: 392401203

Document CLIF prerequisites in README

PiperOrigin-RevId: 335537695