[libc++] Mark __wrap_iter's private constructors as explicit

[• Quuxplusone]

Summary

This is slightly more user-visible than D119894, because the user is
expected to touch __wrap_iter directly. But the affected ctors are
non-public, so the user was never expected to be actually calling them.
And I didn't intentionally omit this from D119894; I just didn't
think of it.

(Btw, I noticed this as a result of researching https://quuxplusone.github.io/blog/2022/03/03/why-isnt-vector-iterator-just-t-star/ )

[jloser]

Seems reasonable to me FWIW.

libcxx/include/__iterator/wrap_iter.h

_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_AFTER_CXX11 reference    operator[](difference_type __n) const _NOEXCEPT
                              "Attempted to subscript an iterator outside its valid range");
         return __i[__n];
     }
 
     _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_AFTER_CXX11 iterator_type base() const _NOEXCEPT {return __i;}
 
 private:
 #if _LIBCPP_DEBUG_LEVEL == 2
-    _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_AFTER_CXX11 __wrap_iter(const void* __p, iterator_type __x) : __i(__x)
+    _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_AFTER_CXX11
+    explicit __wrap_iter(const void* __p, iterator_type __x) : __i(__x)
     {
       if (!__libcpp_is_constant_evaluated())
         __get_db()->__insert_ic(this, __p);
     }
 #else
-    _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_AFTER_CXX11 __wrap_iter(iterator_type __x) _NOEXCEPT : __i(__x) {}
+    _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_AFTER_CXX11
+    explicit __wrap_iter(iterator_type __x) _NOEXCEPT : __i(__x) {}
 #endif
 
     template <class _Up> friend class __wrap_iter;
     template <class _CharT, class _Traits, class _Alloc> friend class basic_string;
     template <class _Tp, class _Alloc> friend class _LIBCPP_TEMPLATE_VIS vector;
     template <class _Tp, size_t> friend class _LIBCPP_TEMPLATE_VIS span;
 };