[libc++][hardening] Add iterator validity checks on unordered containers (#80230)

These are simply null checks, so use `_LIBCPP_ASSERT_NON_NULL`. This
allows us to restore a bunch of the old debug tests. I've extended them
to also cover the const iterators, as those run through different
codepaths than the const ones.

This does the easier (and less important) half of #80212.