libstdc++: Define operator== for hash table iterators [PR115939]

libstdc++: Define operator== for hash table iterators [PR115939]

Currently iterators for unordered containers do not directly define
operator== and operator!= overloads. Instead they rely on the base class
defining them, which is done so that iterator and const_iterator
comparisons work using the same overloads.

However this means a derived-to-base conversion is needed to call those
operators, and PR libstdc++/115939 shows that this can be ambiguous (for
-pedantic) when another overloaded operator could be used after an
implicit conversion.

This change defines operator== and operator!= directly for
_Node_iterator and _Node_const_iterator so that no derived-to-base
conversions are needed. The new overloads just forward to the base class
ones, so the implementation is still shared and doesn't need to be
duplicated.

libstdc++-v3/ChangeLog:

	PR libstdc++/115939
	* include/bits/hashtable_policy.h (_Node_iterator): Add
	operator== and operator!=.
	(_Node_const_iterator): Likewise.
	* testsuite/23_containers/unordered_map/115939.cc: New test.