[re.results.general] Add match_results::const_iterator to index of implementation-defined behavior

Author: AlisdairM

source/regex.tex

@@ -2176,7 +2176,7 @@
       using value_type      = sub_match<BidirectionalIterator>;
       using const_reference = const value_type&;
       using reference       = value_type&;
-      using const_iterator  = @{\impdef}@;
+      using const_iterator  = @\impdefx{type of \tcode{match_results::const_iterator}}@;
       using iterator        = const_iterator;
       using difference_type =
               typename iterator_traits<BidirectionalIterator>::difference_type;