libstdc++: Add _Hashtable::_M_assign for the common case

This adds a convenient _M_assign overload for the common case where the
node generator is the _AllocNode type. Only two places need to call
_M_assign with a _ReuseOrAllocNode node generator, so all the other
calls to _M_assign can use the new overload instead of manually
constructing a node generator.

The _AllocNode::operator(Args&&...) function doesn't need to be a
variadic template. It is only ever called with a single argument of type
const value_type& or value_type&&, so could be simplified. That isn't
done in this commit.

libstdc++-v3/ChangeLog:

	* include/bits/hashtable.h (_Hashtable): Remove typedefs for
	node generators.
	(_Hashtable::_M_assign(_Ht&&)): Add new overload.
	(_Hashtable::operator=(initializer_list<value_type>)): Add local
	typedef for node generator.
	(_Hashtable::_M_assign_elements): Likewise.
	(_Hashtable::operator=(const _Hashtable&)): Use new _M_assign
	overload.
	(_Hashtable(const _Hashtable&)): Likewise.
	(_Hashtable(const _Hashtable&, const allocator_type&)):
	Likewise.
	(_Hashtable(_Hashtable&&, __node_alloc_type&&, false_type)):
	Likewise.
	* include/bits/hashtable_policy.h (_Insert): Remove typedef for
	node generator.

Reviewed-by: François Dumont <fdumont@gcc.gnu.org>