Implement P0513R0 - "Poisoning the Hash"

Summary:
Exactly what the title says.

This patch also adds a `std::hash<nullptr_t>` specialization in C++17, but it was not added by this paper and I can't find the actual paper that adds it.

See http://wg21.link/P0513R0 for more info.

If there are no comments in the next couple of days I'll commit this

Reviewers: mclow.lists, K-ballo, EricWF

Reviewed By: EricWF

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D28938

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@292684 91177308-0d34-0410-b5e6-96231b3b80d8

test/libcxx/test/config.py

@@ -423,15 +423,6 @@ class Configuration(object):
         if not std:
             # Choose the newest possible language dialect if none is given.
             possible_stds = ['c++1z', 'c++14', 'c++11', 'c++03']
-            if self.cxx.type == 'gcc':
-                maj_v, _, _ = self.cxx.version
-                maj_v = int(maj_v)
-                if maj_v < 7:
-                    possible_stds.remove('c++1z')
-                # FIXME: How many C++14 tests actually fail under GCC 5 and 6?
-                # Should we XFAIL them individually instead?
-                if maj_v <= 6:
-                    possible_stds.remove('c++14')
             for s in possible_stds:
                 if self.cxx.hasCompileFlag('-std=%s' % s):
                     std = s