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

It was previously implemented by commit [45d048c20](https://reviews.llvm.org/rG45d048c20440989df2b4e1be1f9343225e7741ab).

Reviewed By: #libc, Quuxplusone, philnik, ldionne

Differential Revision: https://reviews.llvm.org/D118751

libcxx/docs/Status/RangesPaper.csv

@@ -67,7 +67,9 @@ Section,Description,Dependencies,Assignee,Complete
 `[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>`_

libcxx/include/iterator

@@ -165,6 +165,7 @@ 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