libstdc++: Rearrange std::move_iterator helpers in stl_iterator.h

libstdc++: Rearrange std::move_iterator helpers in stl_iterator.h

The __niter_base(move_iterator<I>) overload and __is_move_iterator trait
were originally immediately after the definition of move_iterator. The
addition of C++20 features after move_iterator meant that those helpers
were no longer anywhere near move_iterator.

This change puts them back where they used to be, before all the new
C++20 additions.

libstdc++-v3/ChangeLog:

	* include/bits/stl_iterator.h (__niter_base(move_iterator<I>))
	(__is_move_iterator, __miter_base, _GLIBCXX_MAKE_MOVE_ITERATOR)
	(_GLIBCXX_MAKE_MOVE_IF_NOEXCEPT_ITERATOR): Move earlier in the
	file.