libstdc++: Adjust std::span::iterator to be ADL-proof

libstdc++: Adjust std::span::iterator to be ADL-proof

Because std::span<Incomplete> can be useful, it makes sense to define
std::span<Incomplete>::iterator such that Incomplete is not an
associated class, and so the compiler won't attempt to complete it when
doing ADL for span iterators (including during the definition of
std::span<Incomplete>::const_iterator which checks that iterator
satisfies std::input_or_output_iterator).

We can't make this change for std::vector<Incomplete> because it would
change the mangled name of std::vector<Incomplete>::iterator which would
affect the mangled names of templates and functions written by users.

We can do the same thing for std::basic_stacktrace<Alloc> just so that
Alloc is not an associated class. This is probably less beneficial, as
Alloc can't be incomplete, and using SomeAllocator<Incomplete> as the
allocator parameter doesn't seem useful. But simply making the
stacktrace iterator not use Alloc for ADL lookup seems worthwhile. This
is doable because std::stacktrace is part of C++23 so its ABI isn't
considered stable yet.

libstdc++-v3/ChangeLog:

	* include/std/span (span::__iter_tag): Declare nested type.
	(span::iterator): Use __iter_tag as second template argument.
	* include/std/stacktrace (basic_stacktrace::iterator): Use _Impl
	as second template argument.