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.

llvm-svn: 273839