libstdc++: Specialize std::disable_sized_sentinel_for for std::move_iterator [PR116549]

LWG 3736 added a partial specialization of this variable template for
two std::move_iterator types. This is needed for the case where the
types satisfy std::sentinel_for and are subtractable, but do not model
the semantics requirements of std::sized_sentinel_for.

libstdc++-v3/ChangeLog:

	PR libstdc++/116549
	* include/bits/stl_iterator.h (disable_sized_sentinel_for):
	Define specialization for two move_iterator types, as per LWG
	3736.
	* testsuite/24_iterators/move_iterator/lwg3736.cc: New test.