[libc++][nfc] Test std::unique_ptr self move assignment.

[Mordante]

Summary

The post-conditions for the self move assignment of std::unique_ptr
were changed. This requires no implementation changes. A test was added
to validate the new post-conditions.

Addresses

• LWG-3455: Incorrect Postconditions on unique_ptr move assignment

[Mordante]

Rebase to make sure CI failures aren't due to this patch.

libcxx/docs/Cxx2bStatus.rst

 Library Working Group Issues Status
 ====================================
 
 .. csv-table::
    :file: Cxx2bStatusIssuesStatus.csv
    :header-rows: 1
    :widths: auto
 
-Last Updated: 01-Mar-2021
+Last Updated: 6-June-2021

libcxx/docs/Cxx2bStatusIssuesStatus.csv

 "`3427 <https://wg21.link/LWG3427>`__","``operator<=>(const shared_ptr<T>&, nullptr_t)`` definition ill-formed","November 2020","",""
 "`3428 <https://wg21.link/LWG3428>`__","``single_view``'s in place constructor should be explicit","November 2020","",""
 "`3434 <https://wg21.link/LWG3434>`__","``ios_base`` never reclaims memory for iarray and parray","November 2020","",""
 "`3437 <https://wg21.link/LWG3437>`__","``__cpp_lib_polymorphic_allocator`` is in the wrong header","November 2020","",""
 "`3446 <https://wg21.link/LWG3446>`__","``indirectly_readable_traits`` ambiguity for types with both ``value_type`` and ``element_type``","November 2020","",""
 "`3448 <https://wg21.link/LWG3448>`__","``transform_view``'s sentinel<false> not comparable with ``iterator<true>``","November 2020","",""
 "`3449 <https://wg21.link/LWG3449>`__","take_view and take_while_view's ``sentinel<false>`` not comparable with their const iterator","November 2020","",""
 "`3453 <https://wg21.link/LWG3453>`__","Generic code cannot call ``ranges::advance(i, s)``","November 2020","",""
-"`3455 <https://wg21.link/LWG3455>`__","Incorrect Postconditions on ``unique_ptr`` move assignment","November 2020","",""
+"`3455 <https://wg21.link/LWG3455>`__","Incorrect Postconditions on ``unique_ptr`` move assignment","November 2020","|Nothing To Do|",""
 "`3460 <https://wg21.link/LWG3460>`__","Unimplementable ``noop_coroutine_handle`` guarantees","November 2020","",""
 "`3461 <https://wg21.link/LWG3461>`__","``convertible_to``'s description mishandles cv-qualified void","November 2020","",""
 "`3465 <https://wg21.link/LWG3465>`__","compare_partial_order_fallback requires ``F < E``","November 2020","",""
 "`3466 <https://wg21.link/LWG3466>`__","Specify the requirements for ``promise``/``future``/``shared_future`` consistently","November 2020","",""
 "`3467 <https://wg21.link/LWG3467>`__","``bool`` can't be an integer-like type","November 2020","",""
 "`3472 <https://wg21.link/LWG3472>`__","``counted_iterator`` is missing preconditions","November 2020","",""
 "`3473 <https://wg21.link/LWG3473>`__","Normative encouragement in non-normative note","November 2020","",""
 "`3474 <https://wg21.link/LWG3474>`__","Nesting ``join_views`` is broken because of CTAD","November 2020","",""

libcxx/test/std/utilities/smartptr/unique.ptr/unique.ptr.class/unique.ptr.asgn/move.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
 
+// Self assignement post-conditions are tested.
+// ADDITIONAL_COMPILE_FLAGS: -Wno-self-move
+
 // <memory>
 
 // unique_ptr
 
 // Test unique_ptr move assignment
 
 // test move assignment.  Should only require a MoveConstructible deleter, or if
 //    deleter is a reference, not even that.
assert(A::count == 0);
     s2 = std::move(s1);
     assert(s2.get() == p);
     assert(s1.get() == 0);
     assert(A::count == expect_alive);
     assert(d1.state() == 5);
     assert(d2.state() == 5);
   }
   assert(A::count == 0);
+  {
+    std::unique_ptr<VT> s(newValue<VT>(expect_alive));
+    A* p = s.get();
+    s = std::move(s);
+    assert(A::count == expect_alive);
+    assert(s.get() == p);
+  }
+  assert(A::count == 0);
 }
 
 template <bool IsArray>
 void test_sfinae() {
   typedef typename std::conditional<IsArray, int[], int>::type VT;
   {
     typedef std::unique_ptr<VT> U;
     static_assert(!std::is_assignable<U, U&>::value, "");