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

include/memory

@@ -361,6 +361,7 @@ class shared_ptr
 {
 public:
     typedef T element_type;
+    typedef weak_ptr<T> weak_type; // C++17
 
     // constructors:
     constexpr shared_ptr() noexcept;
@@ -3863,6 +3864,9 @@ class _LIBCPP_TYPE_VIS_ONLY shared_ptr
 {
 public:
     typedef _Tp element_type;
+#if _LIBCPP_STD_VER > 14
+    typedef weak_ptr<_Tp> weak_type;
+#endif
 private:
     element_type*      __ptr_;
     __shared_weak_count* __cntrl_;