[ADT][NFCI]: Fix iterator category for graph iterators with external storage

Set the iterator category for graph iterators to input_iterator_tag when
the visited set is stored externally. In that case we can't provide
multi-pass guarantee, so we should not claim to be a forward iterator.

Fixes: #116400

Author: Maetveis

llvm/include/llvm/ADT/DepthFirstIterator.h

@@ -39,6 +39,7 @@
 #include "llvm/ADT/iterator_range.h"
 #include <iterator>
 #include <optional>
+#include <type_traits>
 #include <utility>
 #include <vector>
 
@@ -84,7 +85,10 @@ template <class GraphT,
           bool ExtStorage = false, class GT = GraphTraits<GraphT>>
 class df_iterator : public df_iterator_storage<SetType, ExtStorage> {
 public:
-  using iterator_category = std::forward_iterator_tag;
+  // When External storage is used we are not multi-pass safe.
+  using iterator_category =
+      std::conditional_t<ExtStorage, std::input_iterator_tag,
+                         std::forward_iterator_tag>;
   using value_type = typename GT::NodeRef;
   using difference_type = std::ptrdiff_t;
   using pointer = value_type *;

llvm/include/llvm/ADT/PostOrderIterator.h

@@ -23,6 +23,7 @@
 #include <iterator>
 #include <optional>
 #include <set>
+#include <type_traits>
 #include <utility>
 
 namespace llvm {
@@ -95,7 +96,10 @@ template <class GraphT,
           bool ExtStorage = false, class GT = GraphTraits<GraphT>>
 class po_iterator : public po_iterator_storage<SetType, ExtStorage> {
 public:
-  using iterator_category = std::forward_iterator_tag;
+  // When External storage is used we are not multi-pass safe.
+  using iterator_category =
+      std::conditional_t<ExtStorage, std::input_iterator_tag,
+                         std::forward_iterator_tag>;
   using value_type = typename GT::NodeRef;
   using difference_type = std::ptrdiff_t;
   using pointer = value_type *;