Diagnose when reverse_iterator is used on path::iterator.

path::iterator isn't a strictly conforming iterator. Specifically
it stashes the current element inside the iterator. This leads to
UB when used with reverse_iterator since it requires the element
to outlive the lifetime of the iterator.

This patch adds a static_assert inside reverse_iterator to disallow
"stashing iterator types", and it tags path::iterator as such a type.

Additionally this patch removes all uses of reverse_iterator<path::iterator>
within the tests.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@300164 91177308-0d34-0410-b5e6-96231b3b80d8

include/iterator

@@ -615,6 +615,14 @@ prev(_BidiretionalIter __x,
     return __x;
 }
 
+
+template <class _Tp, class = void>
+struct __is_stashing_iterator : false_type {};
+
+template <class _Tp>
+struct __is_stashing_iterator<_Tp, typename __void_t<typename _Tp::__stashing_iterator_tag>::type>
+  : true_type {};
+
 template <class _Iter>
 class _LIBCPP_TEMPLATE_VIS reverse_iterator
     : public iterator<typename iterator_traits<_Iter>::iterator_category,
@@ -625,6 +633,11 @@ class _LIBCPP_TEMPLATE_VIS reverse_iterator
 {
 private:
     /*mutable*/ _Iter __t;  // no longer used as of LWG #2360, not removed due to ABI break
+
+    static_assert(!__is_stashing_iterator<_Iter>::value,
+      "The specified iterator type cannot be used with reverse_iterator; "
+      "Using stashing iterators with reverse_iterator causes undefined behavior");
+
 protected:
     _Iter current;
 public: