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

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.

Differential Revision: https://reviews.llvm.org/D120937

Author: Quuxplusone

libcxx/include/__iterator/wrap_iter.h

@@ -136,13 +136,15 @@ class __wrap_iter
 
 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;