libstdc++: Avoid instantiation of _Hash_node before it's needed

This is a step towards restoring support for incomplete types in
unordered containers (PR 53339).

We do not need to instantiate the node type to get its value_type
member, because we know that the value type is the first template
parameter. We can deduce that template argument using a custom trait and
a partial specialization for _Hash_node. If we wanted to support custom
hash node types we could still use typename _Tp::value_type in the
primary template of that trait, but that seems unnecessary.

The other change needed is to defer a static assert at class scope, so
that it is done when the types are complete. We must have a complete
type in the destructor, so we can do it there instead.

libstdc++-v3/ChangeLog:

	* include/bits/hashtable.h: Move static assertion to destructor.
	* include/bits/hashtable_policy.h: Deduce value type from node
	type without instantiating it.

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