[Support] Don't use std::iterator, it's deprecated in C++17

[chandlerc]

LGTM, but please update description when committing to explain that you're also leveraging the facade to flesh out the iterator operations and removing the now unecessary manual implementations.

llvm/trunk/include/llvm/Support/Path.h

 // path class.
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef LLVM_SUPPORT_PATH_H
 #define LLVM_SUPPORT_PATH_H
 
 #include "llvm/ADT/Twine.h"
+#include "llvm/ADT/iterator.h"
 #include "llvm/Support/DataTypes.h"
 #include <iterator>
 
 namespace llvm {
 namespace sys {
 namespace path {
 
 enum class Style { windows, posix, native };

 ///   /          => /
 ///   /foo       => /,foo
 ///   foo/       => foo,.
 ///   /foo/bar   => /,foo,bar
 ///   ../        => ..,.
 ///   C:\foo\bar => C:,/,foo,bar
 /// @endcode
 class const_iterator
-    : public std::iterator<std::input_iterator_tag, const StringRef> {
+    : public iterator_facade_base<const_iterator, std::input_iterator_tag,
+                                  const StringRef> {
   StringRef Path;      ///< The entire path.
   StringRef Component; ///< The current component. Not necessarily in Path.
   size_t    Position;  ///< The iterators current position within Path.
   Style S;             ///< The path style to use.
 
   // An end iterator has Position = Path.size() + 1.
   friend const_iterator begin(StringRef path, Style style);
   friend const_iterator end(StringRef path);
 
 public:
   reference operator*() const { return Component; }
-  pointer   operator->() const { return &Component; }
   const_iterator &operator++();    // preincrement
   bool operator==(const const_iterator &RHS) const;
-  bool operator!=(const const_iterator &RHS) const { return !(*this == RHS); }
 
   /// @brief Difference in bytes between this and RHS.
   ptrdiff_t operator-(const const_iterator &RHS) const;
 };
 
 /// @brief Reverse path iterator.
 ///
 /// This is an input iterator that iterates over the individual components in
 /// \a path in reverse order. The traversal order is exactly reversed from that
 /// of \a const_iterator
 class reverse_iterator
-    : public std::iterator<std::input_iterator_tag, const StringRef> {
+    : public iterator_facade_base<reverse_iterator, std::input_iterator_tag,
+                                  const StringRef> {
   StringRef Path;      ///< The entire path.
   StringRef Component; ///< The current component. Not necessarily in Path.
   size_t    Position;  ///< The iterators current position within Path.
   Style S;             ///< The path style to use.
 
   friend reverse_iterator rbegin(StringRef path, Style style);
   friend reverse_iterator rend(StringRef path);
 
 public:
   reference operator*() const { return Component; }
-  pointer   operator->() const { return &Component; }
   reverse_iterator &operator++();    // preincrement
   bool operator==(const reverse_iterator &RHS) const;
-  bool operator!=(const reverse_iterator &RHS) const { return !(*this == RHS); }
 
   /// @brief Difference in bytes between this and RHS.
   ptrdiff_t operator-(const reverse_iterator &RHS) const;
 };
 
 /// @brief Get begin iterator over \a path.
 /// @param path Input path.
 /// @returns Iterator initialized with the first component of \a path.