unordered_map: Match emplace_hint logic when _LIBCPP_DEBUG, NFC

[dexonsmith]

Summary

When !defined(_LIBCPP_DEBUG), unordered_map::emplace_hint() forwards
to emplace(). Do the same when defined(_LIBCPP_DEBUG).

This has no real functionality change, just unifies the logic as a
prep commit.

[EricWF]

This no longer seems relevant.

include/unordered_map

#ifndef _LIBCPP_HAS_NO_VARIADICS
     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"
                 " referring to this unordered_map");
-            return __table_.__emplace_unique(_VSTD::forward<_Args>(__args)...).first;
+            return emplace(_VSTD::forward<_Args>(__args)...).first;
         }
 #else
         iterator emplace_hint(const_iterator, _Args&&... __args)
             {return emplace(_VSTD::forward<_Args>(__args)...).first;}
 #endif
 #endif  // _LIBCPP_HAS_NO_VARIADICS
 #endif  // _LIBCPP_HAS_NO_RVALUE_REFERENCES
     _LIBCPP_INLINE_VISIBILITY