stop using deprecated std::iterator

Summary:
std::iterator was deprecated in C++17, so define the iterator trait
properties directly. This fixes warnings during compilation with Clang
on Windows.

Reviewed By: yfeldblum

Differential Revision: D23353495

fbshipit-source-id: 7c6c0b7fb175251260e5b519ce98bd679b6c99c4

folly/container/detail/F14Policy.h

@@ -412,13 +412,17 @@ struct BasePolicy
 
 // BaseIter is a convenience for concrete set and map implementations
 template <typename ValuePtr, typename Item>
-class BaseIter : public std::iterator<
-                     std::forward_iterator_tag,
-                     std::remove_const_t<
-                         typename std::pointer_traits<ValuePtr>::element_type>,
-                     std::ptrdiff_t,
-                     ValuePtr,
-                     decltype(*std::declval<ValuePtr>())> {
+class BaseIter {
+ private:
+  using pointee = typename std::pointer_traits<ValuePtr>::element_type;
+
+ public:
+  using iterator_category = std::forward_iterator_tag;
+  using value_type = std::remove_const_t<pointee>;
+  using difference_type = std::ptrdiff_t;
+  using pointer = ValuePtr;
+  using reference = pointee&;
+
  protected:
   using Chunk = F14Chunk<Item>;
   using ChunkPtr =