Extract key to avoid preemptive mallocs in insert/emplace in associative containers

Summary: This patch applies Duncan's work on __hash_table to __tree.

Reviewers: mclow.lists, dexonsmith

Subscribers: dexonsmith, cfe-commits

Differential Revision: http://reviews.llvm.org/D18637

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@266491 91177308-0d34-0410-b5e6-96231b3b80d8

include/__tuple

@@ -95,8 +95,6 @@ get(const tuple<_Tp...>&&) _NOEXCEPT;
 
 // pair specializations
 
-template <class _T1, class _T2> struct _LIBCPP_TYPE_VIS_ONLY pair;
-
 template <class _T1, class _T2> struct __tuple_like<pair<_T1, _T2> > : true_type {};
 
 template <size_t _Ip, class _T1, class _T2>