Those methods are making a double lookup in case of insertion, they can
perform only one.
PR libstdc++/95079
* include/bits/hashtable_policy.h (_Insert_base<>::try_emplace): New.
* include/bits/unordered_map.h (unordered_map<>::try_emplace): Adapt.
(unordered_map<>::insert_or_assign): Adapt.