libstdc++: Review _Local_iterator/_Local_const_iterator implementations.

libstdc++: Review _Local_iterator/_Local_const_iterator implementations.

_Local_iterator_base inherits _Node_iterator_base and so share the same
comparison operators. It avoids to expose special method _M_curr for debug
mode to compare such iterators.

libstdc++-v3/ChangeLog:

	* include/bits/hashtable_policy.h (_Node_iterator_base()): New.
	(operator==(const _Node_iterator_base&, const _Node_iterator_base&)):
	Make hidden friend.
	(operator!=(const _Node_iterator_base&, const _Node_iterator_base&)):
	Make hidden friend.
	(_Local_iterator_base<>): Inherits _Node_iterator_base.
	(_Local_iterator_base<>::_M_cur): Remove.
	(_Local_iterator_base<>::_M_curr()): Remove.
	(operator==(const _Local_iterator_base&, const _Local_iterator_base&)):
	Remove.
	(operator!=(const _Local_iterator_base&, const _Local_iterator_base&)):
	Remove.
	* include/debug/unordered_map (unordered_map<>::_M_invalidate): Adapt.
	(unordered_multimap<>::_M_invalidate): Adapt.
	* include/debug/unordered_set (unordered_set<>::_M_invalidate): Adapt.
	(unordered_multiset<>::_M_invalidate): Adapt.