unordered_map: Use __hash_table::__emplace_unique(), NFC

[dexonsmith]

Summary

Instead of duplicating code in unordered_map::emplace(), use
hash_table::emplace_unique().

[EricWF]

This is very subtly broken. The requirements in the standard for emplace are

[unord.req] Table 102

a_eq.emplace(args)
Requires: : value_type shall be EmplaceConstructible into X from args.

Unfortunately __hash_table doesn't know that unordered_map has this "special" value_type, so it can't actually extract and construct the correct value_type from it. I'm going to change
this over the weekend. Once I've done some prep work this patch should be ready to go.

[EricWF]

All of the prep work has landed. This patch is no longer needed though.

[EricWF]

I think this has been landed (or is no longer relevent) after all of the recent changes.

include/unordered_map

#endif  // _LIBCPP_HAS_NO_GENERALIZED_INITIALIZERS
     const_iterator cbegin() const _NOEXCEPT {return __table_.begin();}
     _LIBCPP_INLINE_VISIBILITY
     const_iterator cend()   const _NOEXCEPT {return __table_.end();}
 
 #ifndef _LIBCPP_HAS_NO_RVALUE_REFERENCES
 #ifndef _LIBCPP_HAS_NO_VARIADICS
 
     template <class... _Args>
-        pair<iterator, bool> emplace(_Args&&... __args);
+    pair<iterator, bool> emplace(_Args&&... __args)
+        {return __table_.__emplace_unique(_VSTD::forward<_Args>(__args)...);}
 
     template <class... _Args>
         _LIBCPP_INLINE_VISIBILITY
 #if _LIBCPP_DEBUG_LEVEL >= 2
         iterator emplace_hint(const_iterator __p, _Args&&... __args)
         {
             _LIBCPP_ASSERT(__get_const_db()->__find_c_from_i(&__p) == this,
                 "unordered_map::emplace_hint(const_iterator, args...) called with an iterator not"
unordered_map<_Key, _Tp, _Hash, _Pred, _Alloc>::__construct_node(_A0&& __a0,
     __node_traits::construct(__na, _VSTD::addressof(__h->__value_),
                              _VSTD::forward<_A0>(__a0), _VSTD::forward<_A1>(__a1),
                              _VSTD::forward<_Args>(__args)...);
     __h.get_deleter().__first_constructed = true;
     __h.get_deleter().__second_constructed = true;
     return __h;
 }
 
-template <class _Key, class _Tp, class _Hash, class _Pred, class _Alloc>
-template <class... _Args>
-pair<typename unordered_map<_Key, _Tp, _Hash, _Pred, _Alloc>::iterator, bool>
-unordered_map<_Key, _Tp, _Hash, _Pred, _Alloc>::emplace(_Args&&... __args)
-{
-    __node_holder __h = __construct_node(_VSTD::forward<_Args>(__args)...);
-    pair<iterator, bool> __r = __table_.__node_insert_unique(__h.get());
-    if (__r.second)
-        __h.release();
-    return __r;
-}
-
 #endif  // _LIBCPP_HAS_NO_VARIADICS
 #endif  // _LIBCPP_HAS_NO_RVALUE_REFERENCES
 
 template <class _Key, class _Tp, class _Hash, class _Pred, class _Alloc>
 typename unordered_map<_Key, _Tp, _Hash, _Pred, _Alloc>::__node_holder
 unordered_map<_Key, _Tp, _Hash, _Pred, _Alloc>::__construct_node_with_key(const key_type& __k)
 {
     __node_allocator& __na = __table_.__node_alloc();