Add a new hash class in __gnu_ext for the extension containers. There

are two motivations for this.

First, this allows users who are specializing __gnu_ext::hash to
continue doing so without changing their code.

Second, SGI specifies hash overloads for char* and const char* that
perform a hash of the string, not of the pointer.

In order to support this, the hashing code for string is factored out.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@136539 91177308-0d34-0410-b5e6-96231b3b80d8

include/ext/__hash

@@ -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_VISIBLE hash : public std::hash<T>
+    { };
+
+template <> struct _LIBCPP_VISIBLE 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_VISIBLE 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