MDEV-20297 Support C++11 range-based for loop for List<T>

New iterator has the fastest possible implementation: just moves one pointer.
It's faster that List_iterator and List_iterator_fast: both do more on increment.

Overall patch brings:
1) work compile times
2) possibly(!) worse debug build performance
3) definitely better optimized build performance
4) ability to write less code
5) ability to write less bug-prone code

sql/sql_list.h

@@ -21,6 +21,7 @@
 #endif
 
 #include "sql_alloc.h"
+#include <iterator>
 
 /**
   Simple intrusive linked list.
@@ -532,6 +533,51 @@ public:
     List<T> *res= new (mem_root) List<T>;
     return res == NULL || res->push_back(first, mem_root) ? NULL : res;
   }
+
+  class Iterator;
+  using value_type= T;
+  using iterator= Iterator;
+  using const_iterator= const Iterator;
+
+  Iterator begin() const { return Iterator(first); }
+  Iterator end() const { return Iterator(); }
+
+  class Iterator
+  {
+  public:
+    using iterator_category= std::forward_iterator_tag;
+    using value_type= T;
+    using difference_type= std::ptrdiff_t;
+    using pointer= T *;
+    using reference= T &;
+
+    Iterator(list_node *p= &end_of_list) : node{p} {}
+
+    Iterator &operator++()
+    {
+      DBUG_ASSERT(node != &end_of_list);
+
+      node= node->next;
+      return *this;
+    }
+
+    T operator++(int)
+    {
+      Iterator tmp(*this);
+      operator++();
+      return tmp;
+    }
+
+    T &operator*() { return *static_cast<T *>(node->info); }
+
+    bool operator!=(const typename List<T>::iterator &rhs)
+    {
+      return node != rhs.node;
+    }
+
+  private:
+    list_node *node{&end_of_list};
+  };
 };