[libcxx][iterator] adds std::indirectly_readable and std::indirectly_writable

Implements parts of:
    * P0896R4 The One Ranges Proposal`

Depends on D99873.

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D100073

libcxx/test/std/strings/basic.string/string.iterators/iterator_concept_conformance.compile.pass.cpp

@@ -0,0 +1,27 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+// UNSUPPORTED: libcpp-no-concepts
+// UNSUPPORTED: gcc-10
+
+// iterator, const_iterator, reverse_iterator, const_reverse_iterator
+
+#include <string>
+
+#include <iterator>
+
+using iterator = std::string::iterator;
+using const_iterator = std::string::const_iterator;
+using value_type = iterator::value_type;
+
+static_assert(std::indirectly_readable<iterator>);
+static_assert(std::indirectly_writable<iterator, value_type>);
+
+static_assert(std::indirectly_readable<const_iterator>);
+static_assert(!std::indirectly_writable<const_iterator, value_type>);