Add libc++ and libc++abi to third-party.

third_party/libcxx/include/ext/__hash

Index: third_party/libcxx/include/ext/__hash
===================================================================
--- third_party/libcxx/include/ext/__hash	(revision 0)
+++ third_party/libcxx/include/ext/__hash	(revision 0)
@@ -0,0 +1,46 @@
+// -*- C++ -*-
+//===------------------------- hash_set ------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP_EXT_HASH
+#define _LIBCPP_EXT_HASH
+
+#pragma GCC system_header
+
+#include <string>
+#include <cstring>
+
+namespace __gnu_cxx {
+using namespace std;
+
+template <typename T> struct _LIBCPP_TYPE_VIS_ONLY hash : public std::hash<T>
+    { };
+
+template <> struct _LIBCPP_TYPE_VIS_ONLY hash<const char*>
+    : public unary_function<const char*, size_t>
+{
+    _LIBCPP_INLINE_VISIBILITY
+    size_t operator()(const char *__c) const _NOEXCEPT
+    {
+        return __do_string_hash(__c, __c + strlen(__c));
+    }
+};
+
+template <> struct _LIBCPP_TYPE_VIS_ONLY hash<char *>
+    : public unary_function<char*, size_t>
+{
+    _LIBCPP_INLINE_VISIBILITY
+    size_t operator()(char *__c) const _NOEXCEPT
+    {
+        return __do_string_hash<const char *>(__c, __c + strlen(__c));
+    }
+};
+}
+
+#endif  // _LIBCPP_EXT_HASH