Revert "P0503R0, adopted in Issaquah, rewords some requirements on nullptr_t...

Revert "P0503R0, adopted in Issaquah, rewords some requirements on nullptr_t and istream_iterator. No code changes were needed, but I updated a few tests.  Also resolved P0509 and P0521, which required no changes to the library or tests."

This reverts commit r286884, because it breaks the Xcode 7 builders:

  http://lab.llvm.org:8080/green/job/clang-stage1-cmake-RA-expensive/1583

Here is a PR that tracks the issue:
https://llvm.org/bugs/show_bug.cgi?id=31016

llvm-svn: 287004