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.

libstdc++-v3/include/std/span

@@ -123,6 +123,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	using __is_compatible_ref
 	  = __is_array_convertible<_Type, remove_reference_t<_Ref>>;
 
+      // Nested type so that _Type is not an associated class of iterator.
+      struct __iter_tag;
+
     public:
       // member types
       using element_type           = _Type;
@@ -133,7 +136,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       using const_pointer          = const _Type*;
       using reference              = element_type&;
       using const_reference        = const element_type&;
-      using iterator = __gnu_cxx::__normal_iterator<pointer, span>;
+      using iterator = __gnu_cxx::__normal_iterator<pointer, __iter_tag>;
       using reverse_iterator       = std::reverse_iterator<iterator>;
 #if __cplusplus > 202002L
       using const_iterator         = std::const_iterator<iterator>;

libstdc++-v3/include/std/stacktrace

@@ -179,13 +179,15 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     {
       using _AllocTraits = allocator_traits<_Allocator>;
       using uintptr_t = __UINTPTR_TYPE__;
+      struct _Impl;
 
     public:
       using value_type = stacktrace_entry;
       using const_reference = const value_type&;
       using reference = value_type&;
       using const_iterator
-	= __gnu_cxx::__normal_iterator<value_type*, basic_stacktrace>;
+	= __gnu_cxx::__normal_iterator<typename _AllocTraits::const_pointer,
+				       _Impl>;
       using iterator = const_iterator;
       using reverse_iterator = std::reverse_iterator<iterator>;
       using const_reverse_iterator = std::reverse_iterator<const_iterator>;