libstdc++: Define converting assignment operator for std::move_iterator

libstdc++: Define converting assignment operator for std::move_iterator

As clarified by LWG 3265, std::move_iterator is supposed to have an
assignment operator that converts from a different specialization of
std::move_iterator, which performs an assignment. That has always been
missing from libstdc++, so assigning a different type actually performs
a converting construction, then an assignment. This is non-conforming
for the (fairly contrived) case where the converting assignment is
well-formed but the converting construction is not.

	* include/bits/stl_iterator.h (move_iterator::operator=): Define.
	* testsuite/24_iterators/move_iterator/dr3265.cc: New test.