…storage

Set the iterator category for graph iterators to input_iterator_tag when
the visited set is stored externally. In that case we can't provide
multi-pass guarantee, so we should not claim to be a forward iterator.

Fixes: llvm#116400

Check that using the same iterators for multiple passes produces the
same result, as required by the C++ standard for forward iterators.

Add a static_asserts for the iterator_category of Graph iterators
(BreadthFirstIterator, DepthFirstIterator, PostOrderIterator).