Recommit r286884: P0503R0, adopted in Issaquah, rewords some requirements on...

Recommit r286884: 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 patch was reverted due to llvm.org/PR31016. There is a bug in Clang 3.7
which causes default.pass.cpp to fails. That test is now marked as XFAIL for that
clang version.

This patch was originally authored by Marshall Clow.

llvm-svn: 289708