[ADT] Relax iterator constraints on all_equal (#106400)

The previous `all_equal` implementation contained `Begin + 1`, which
implicitly requires `Begin` to model the
[random_access_iterator](https://en.cppreference.com/w/cpp/iterator/random_access_iterator)
concept due to the usage of the `+` operator. By swapping this out with
`std::next`, this method can be used with weaker iterator concepts, such
as
[forward_iterator](https://en.cppreference.com/w/cpp/iterator/forward_iterator).

---------

Co-authored-by: Jakub Kuderski <kubakuderski@gmail.com>

Author: aws-taylor

llvm/include/llvm/ADT/STLExtras.h

@@ -2062,7 +2062,7 @@ bool equal(L &&LRange, R &&RRange, BinaryPredicate P) {
 template <typename R> bool all_equal(R &&Range) {
   auto Begin = adl_begin(Range);
   auto End = adl_end(Range);
-  return Begin == End || std::equal(Begin + 1, End, Begin);
+  return Begin == End || std::equal(std::next(Begin), End, Begin);
 }
 
 /// Returns true if all Values in the initializer lists are equal or the list

llvm/unittests/ADT/STLExtrasTest.cpp

@@ -15,6 +15,7 @@
 #include <climits>
 #include <cstddef>
 #include <initializer_list>
+#include <iterator>
 #include <list>
 #include <tuple>
 #include <type_traits>
@@ -831,6 +832,26 @@ TEST(STLExtrasTest, AllEqual) {
   EXPECT_FALSE(all_equal(V));
 }
 
+// Test to verify that all_equal works with a container that does not
+// model the random access iterator concept.
+TEST(STLExtrasTest, AllEqualNonRandomAccess) {
+  std::list<int> V;
+  static_assert(!std::is_convertible_v<
+                std::iterator_traits<decltype(V)::iterator>::iterator_category,
+                std::random_access_iterator_tag>);
+  EXPECT_TRUE(all_equal(V));
+
+  V.push_back(1);
+  EXPECT_TRUE(all_equal(V));
+
+  V.push_back(1);
+  V.push_back(1);
+  EXPECT_TRUE(all_equal(V));
+
+  V.push_back(2);
+  EXPECT_FALSE(all_equal(V));
+}
+
 TEST(STLExtrasTest, AllEqualInitializerList) {
   EXPECT_TRUE(all_equal({1}));
   EXPECT_TRUE(all_equal({1, 1}));