fixed reverse_iterator

Summary:
The reverse iterator was incorrectly implemented.

Added test case fails when ran without the fix:
```
xplat/folly/container/test/span_test.cpp:333: Failure
Expected equality of these values:
  2
  *it++
    Which is: 0

xplat/folly/container/test/span_test.cpp:334: Failure
Expected equality of these values:
  1
  *it++
    Which is: 2

xplat/folly/container/test/span_test.cpp:335: Failure
Expected equality of these values:
  it
    Which is: 8-byte object <18-FF 4A-8A FF-7F 00-00>
  obj.rend()
    Which is: 8-byte object <28-FF 4A-8A FF-7F 00-00>
```

Reviewed By: ilvokhin

Differential Revision: D70909341

fbshipit-source-id: 61efb3b87761dc1f13dae75964b06bee798803f4

Author: Fredrik Olsson

folly/container/span.h

@@ -180,10 +180,10 @@ class span {
   constexpr iterator begin() const noexcept { return data_; }
   constexpr iterator end() const noexcept { return data_ + size(); }
   constexpr reverse_iterator rbegin() const noexcept {
-    return std::make_reverse_iterator(begin());
+    return std::make_reverse_iterator(end());
   }
   constexpr reverse_iterator rend() const noexcept {
-    return std::make_reverse_iterator(end());
+    return std::make_reverse_iterator(begin());
   }
 
   constexpr reference front() const {

folly/container/test/span_test.cpp

@@ -324,6 +324,17 @@ TYPED_TEST_P(SpanTest, fix_as_bytes) {
   EXPECT_EQ(20, wbytes.size());
 }
 
+TYPED_TEST_P(SpanTest, reverse_iterator) {
+  using span = typename TypeParam::template apply<int, size_t(-1)>;
+
+  int data[2] = {1, 2};
+  auto const obj = span{data};
+  auto it = obj.rbegin();
+  EXPECT_EQ(2, *it++);
+  EXPECT_EQ(1, *it++);
+  EXPECT_EQ(it, obj.rend());
+}
+
 namespace fallback_span_ctad {
 
 namespace fallback = folly::detail::fallback_span;
@@ -390,6 +401,7 @@ REGISTER_TYPED_TEST_SUITE_P(
     , fix_static_subspan
     , fix_dynamic_subspan
     , fix_as_bytes
+    , reverse_iterator
     );
 // clang-format on