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