Update synposis in <memory> to show move semantics for weak_ptr; add tests for already existing move semantics. Mark LWG issues #2315 (no changes needed), 2316 (move semantics for weak_ptr), 2252 (previous commit) and 2271 (previous commit) as complete.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@202931 91177308-0d34-0410-b5e6-96231b3b80d8

test/utilities/memory/util.smartptr/util.smartptr.weak/util.smartptr.weak.assign/weak_ptr.pass.cpp

@@ -59,4 +59,20 @@ int main()
     }
     assert(B::count == 0);
     assert(A::count == 0);
+
+    {
+        const std::shared_ptr<A> ps(new A);
+        std::weak_ptr<A> pA(ps);
+        {
+            std::weak_ptr<A> pB;
+            pB = std::move(pA);
+            assert(B::count == 1);
+            assert(A::count == 1);
+            assert(pB.use_count() == 1);
+        }
+        assert(B::count == 1);
+        assert(A::count == 1);
+    }
+    assert(B::count == 0);
+    assert(A::count == 0);
 }