fix deprecation of std::iterator (#7332)

When compiling cvc5 with clang-13, it will emit lots of warnings of usages of `std::iterator` as it's deprecated since C++17. 
The recommended implementation of iterators is to manually define the following five types: 
```
template< class Iter >
struct iterator_traits;

difference_type	        Iter::difference_type
value_type	        Iter::value_type
pointer	                Iter::pointer
reference	        Iter::reference
iterator_category	Iter::iterator_category

```
And the iterator-related types could be accessed by for example `typename std::iterator_traits<Iter>::value_type value`. 


This pull request performs the fix, and the program is semantically equivalent before and after the fix. 

References: 
https://en.cppreference.com/w/cpp/iterator/iterator_traits 
https://en.cppreference.com/w/cpp/iterator/iterator 

1 files changed, 1 insertions, 1 deletions

diff --git a/src/context/cdlist.h b/src/context/cdlist.h
index 007acc64d..997dbc924 100644
--- a/src/context/cdlist.h
+++ b/src/context/cdlist.h
@@ -384,7 +384,7 @@ protected:
 
     // FIXME we support operator--, but do we satisfy all the
     // requirements of a bidirectional iterator ?
-    typedef std::input_iterator_tag iterator_category;
+    typedef std::bidirectional_iterator_tag iterator_category;
     typedef T value_type;
     typedef std::ptrdiff_t difference_type;
     typedef const T* pointer;