libstdc++: Limit allocation on iterator insertion in Hashtable [PR 96088]

libstdc++: Limit allocation on iterator insertion in Hashtable [PR 96088]

When inserting into unordered_multiset or unordered_multimap first instantiate
the node to store and compute the hash code from it to avoid a potential
intermediate key_type instantiation.

When inserting into unordered_set or unordered_map check if invoking the hash
functor with container key_type is noexcept and invoking the same hash functor
with key part of the iterator value_type can throw. In this case create a
temporary key_type instance at Hashtable level and use it to compute the hash
code. This temporary instance is moved to the final storage location if needed.

libstdc++-v3/ChangeLog:

	PR libstdc++/96088
	* include/bits/hashtable_policy.h (_Select2nd): New.
	(_NodeBuilder<>): New.
	(_ReuseOrAllocNode<>::operator()): Use variadic template args.
	(_AllocNode<>::operator()): Likewise.
	* include/bits/hashtable.h
	(_Hashtable<>::__node_builder_t): New.
	(_Hashtable<>::_M_insert_unique<>(_Kt&&, _Arg&&, const _NodeGenerator&)):
	 New.
	(_Hashtable<>::_S_forward_key): New.
	(_Hashtable<>::_M_insert): Use latter.
	(_Hashtable<>::_M_insert(const_iterator, _Arg&&, const _NodeGenerator&, false_type)):
	Instantiate node first, compute hash code second.
	* testsuite/23_containers/unordered_map/96088.cc: New test.
	* testsuite/23_containers/unordered_multimap/96088.cc: New test.
	* testsuite/23_containers/unordered_multiset/96088.cc: New test.
	* testsuite/23_containers/unordered_set/96088.cc: New test.
	* testsuite/util/replacement_memory_operators.h
	(counter::_M_increment): New.
	(counter::_M_decrement): New.
	(counter::reset()): New.