[reland][libc] Add reverse_iterator comparisons (#86188)

This is a reland of #86147 but with a proper `base()` function.

https://en.cppreference.com/w/cpp/iterator/reverse_iterator/operator_cmp

Author: gchatelet

libc/src/__support/CPP/iterator.h

@@ -20,13 +20,16 @@ namespace cpp {
 template <typename T> struct iterator_traits;
 template <typename T> struct iterator_traits<T *> {
   using reference = T &;
+  using value_type = T;
 };
 
 template <typename Iter> class reverse_iterator {
   Iter current;
 
 public:
   using reference = typename iterator_traits<Iter>::reference;
+  using value_type = typename iterator_traits<Iter>::value_type;
+  using iterator_type = Iter;
 
   LIBC_INLINE reverse_iterator() : current() {}
   LIBC_INLINE constexpr explicit reverse_iterator(Iter it) : current(it) {}
@@ -38,6 +41,38 @@ template <typename Iter> class reverse_iterator {
   LIBC_INLINE constexpr explicit reverse_iterator(const Other &it)
       : current(it) {}
 
+  LIBC_INLINE friend constexpr bool operator==(const reverse_iterator &lhs,
+                                               const reverse_iterator &rhs) {
+    return lhs.base() == rhs.base();
+  }
+
+  LIBC_INLINE friend constexpr bool operator!=(const reverse_iterator &lhs,
+                                               const reverse_iterator &rhs) {
+    return lhs.base() != rhs.base();
+  }
+
+  LIBC_INLINE friend constexpr bool operator<(const reverse_iterator &lhs,
+                                              const reverse_iterator &rhs) {
+    return lhs.base() > rhs.base();
+  }
+
+  LIBC_INLINE friend constexpr bool operator<=(const reverse_iterator &lhs,
+                                               const reverse_iterator &rhs) {
+    return lhs.base() >= rhs.base();
+  }
+
+  LIBC_INLINE friend constexpr bool operator>(const reverse_iterator &lhs,
+                                              const reverse_iterator &rhs) {
+    return lhs.base() < rhs.base();
+  }
+
+  LIBC_INLINE friend constexpr bool operator>=(const reverse_iterator &lhs,
+                                               const reverse_iterator &rhs) {
+    return lhs.base() <= rhs.base();
+  }
+
+  LIBC_INLINE constexpr iterator_type base() const { return current; }
+
   LIBC_INLINE constexpr reference operator*() const {
     Iter tmp = current;
     return *--tmp;