libstdc++: Fix std::unordered_set::emplace optimization [PR117686]

libstdc++: Fix std::unordered_set::emplace optimization [PR117686]

The __is_key_type specialization that matches a pair<key_type, T>
argument is intended for std::unordered_map, not for
std::unordered_set<std::pair<K,T>>.

This uses a pair<const Args&...> as the template argument for
__is_key_type, so that it won't match a set's key_type.

libstdc++-v3/ChangeLog:

	PR libstdc++/117686
	* include/bits/hashtable.h (_Hashtable::_M_emplace_uniq):
	Adjust usage of __is_key_type to avoid false positive.
	* testsuite/23_containers/unordered_set/insert/117686.cc:
	New test.