[Bug libstdc++/97876] stop_token header doesn't compile on clang-8 with -std=c++20

[eric.niebler at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97876

--- Comment #3 from Eric Niebler <eric.niebler at gmail dot com> ---
It seems like GitHub actions uses the latest libstdc++ by default when testing
even with old (e.g., clang-4) toolsets. That seems busted, but regardless, this
is now breaking _all_ my Linux clang tests for anything less than clang-9,
which is unfortunate.