PR libstdc++/92124 fix incorrect unordered container move assignment

PR libstdc++/92124 fix incorrect unordered container move assignment

	* include/bits/hashtable.h (_Hashtable<>::__alloc_node_gen_t): New
	template alias.
	(_Hashtable<>::__fwd_value_for): New.
	(_Hashtable<>::_M_assign_elements<>): Remove _NodeGenerator template
	parameter.
	(_Hashtable<>::_M_assign<>): Add _Ht template parameter.
	(_Hashtable<>::operator=(const _Hashtable<>&)): Adapt.
	(_Hashtable<>::_M_move_assign): Adapt. Replace std::move_if_noexcept
	with std::move.
	(_Hashtable<>::_Hashtable(const _Hashtable&)): Adapt.
	(_Hashtable<>::_Hashtable(const _Hashtable&, const allocator_type&)):
	Adapt.
	(_Hashtable<>::_Hashtable(_Hashtable&&, const allocator_type&)):
	Adapt.
	* testsuite/23_containers/unordered_set/92124.cc: New.

From-SVN: r280028