...for anything that already had _LIBCPP_NODISCARD_EXT or
nodiscard in libc++ has had multi-layered complexity for quite some
time, requiring users to know that they need to opt into a lot of what
we mark as nodiscard. There are also many knobs: an opt-in mechanism
(that users can easily overlook) and three opt-out mechanisms, which can
lead to confused users.
This patch does not break any code that wasn't already buggy or
questionable to begin with.