Implement P0163r0. Add shared_ptr::weak_type.

This patch adds the weak_type typedef in shared_ptr. It is available in
C++17 and newer.

This patch also updates the _LIBCPP_STD_VER and TEST_STD_VER macros to
have the value of 16, since 2016 is the current year.


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@273839 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/utilities/memory/util.smartptr/util.smartptr.shared/types.pass.cpp

@@ -13,14 +13,20 @@
 // {
 // public:
 //     typedef T element_type;
+//     typedef weak_ptr<T> weak_type; // C++17
 //     ...
 // };
 
 #include <memory>
 
+#include "test_macros.h"
+
 struct A;  // purposefully incomplete
 
 int main()
 {
     static_assert((std::is_same<std::shared_ptr<A>::element_type, A>::value), "");
+#if TEST_STD_VER > 14
+    static_assert((std::is_same<std::shared_ptr<A>::weak_type, std::weak_ptr<A>>::value), "");
+#endif
 }