Add two new macros: _LIBCPP_NODISCARD_AFTER_CXX17 and _LIBCPP_CONSTEXPR_AFTER_CXX17, along with a way to turn off the NODISCARD one: _LIBCPP_DISABLE_NODISCARD_AFTER_CXX17. No one is using these yet, but we will be ... soon

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@318208 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed