[libc++][ranges][NFC] Add contiguous_iterator_tag to the <iterator> synopsis

[var-const]

Summary

It was previously implemented by commit 45d048c20.

libcxx/docs/Status/RangesPaper.csv

 | indirectly_copyable
 | indirectly_copyable_storable",[iterator.concepts],Zoe Carver,In progress
 `[common.alg.req] <https://wg21.link/common.alg.req>`_: pt. 2,`indirectly_swappable <https://llvm.org/D105304>`_,"| [iterator.concepts]
 | [iterator.cust.swap]",Zoe Carver,✅
 `[common.alg.req] <https://wg21.link/common.alg.req>`_: pt. 3,`indirectly_comparable <https://llvm.org/D116268>`_,[projected],Nikolas Klauser,✅
 `[common.alg.req] <https://wg21.link/common.alg.req>`_: pt. 4,"| permutable
 | mergeable
 | sortable",[iterator.concepts],Unassigned,Not started
-`[std.iterator.tags] <https://wg21.link/std.iterator.tags>`_,,[iterator.traits],Unassigned,Not started
+`[std.iterator.tags] <https://wg21.link/std.iterator.tags>`_,"`contiguous_iterator_tag <https://reviews.llvm.org/rG45d048c20440989df2b4e1be1f9343225e7741ab>`_
+| `iterator_concept specialization for pointers <https://reviews.llvm.org/rG45d048c20440989df2b4e1be1f9343225e7741ab>`_
+",[iterator.traits],Eric Fiselier,✅
 `[range.iter.ops] <https://wg21.link/range.iter.ops>`_,"| `ranges::advance <https://llvm.org/D101922>`_
 | `ranges::distance <https://llvm.org/D102789>`_
 | `ranges::next <https://llvm.org/D102563>`_
 | `ranges::prev <https://llvm.org/D102564>`_",[iterator.concepts],Christopher Di Bella and Arthur O'Dwyer,✅
 `[predef.iterators] <https://wg21.link/predef.iterators>`_,"
 | Updates to reverse_iterator
 | Updates to back_insert_iterator
 | Updates to front_insert_iterator

libcxx/include/iterator

struct iterator                                            // deprecated in C++17
     typedef Category  iterator_category;
 };
 
 struct input_iterator_tag  {};
 struct output_iterator_tag {};
 struct forward_iterator_tag       : public input_iterator_tag         {};
 struct bidirectional_iterator_tag : public forward_iterator_tag       {};
 struct random_access_iterator_tag : public bidirectional_iterator_tag {};
+struct contiguous_iterator_tag    : public random_access_iterator_tag {};
 
 // 27.4.3, iterator operations
 template <class InputIterator, class Distance>  // constexpr in C++17
   constexpr void advance(InputIterator& i, Distance n);
 
 template <class InputIterator>  // constexpr in C++17
   constexpr typename iterator_traits<InputIterator>::difference_type
     distance(InputIterator first, InputIterator last);