N3644 Comparing value-initialized iterators

[Jonathan Wakely <jwakely dot gcc at gmail dot com>]

http://www.open-std.org/JTC1/sc22/WG21/docs/papers/2013/n3644.pdf
makes it OK to compare two value-initialized iterators, and is one of
the few C++14 library features not implemented on trunk.

I haven't checked, but my assumption is that the only change needed to
the library is to allow examples like this in Debug Mode:

  std::vector<int>::iterator i{}; // value init
  i == i;

Currently this fails because the iterator is singular, but we need to
allow it, *without* allowing this:

  std::vector<int>::iterator i;  // default init
  i == i;

I'm not sure how to do this. I thought about trying to play games with
uninitialized members that would be zero-init'd by value-init and left
uninit'd by default-init, but I gave up.