Installation of CityHash by Craig Silverstein

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@146329 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/string b/include/string
index 4457e4a..620e6f8 100644
--- a/include/string
+++ b/include/string
@@ -3916,7 +3916,7 @@
 size_t _LIBCPP_INLINE_VISIBILITY __do_string_hash(_Ptr __p, _Ptr __e)
 {
     typedef typename iterator_traits<_Ptr>::value_type value_type;
-    return __murmur2<size_t>()(__p, (__e-__p)*sizeof(value_type));
+    return __murmur2_or_cityhash<size_t>()(__p, (__e-__p)*sizeof(value_type));
 }
 
 template<class _CharT, class _Traits, class _Allocator>