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

[eric.niebler at gmail dot com]

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

--- Comment #5 from Eric Niebler <eric.niebler at gmail dot com> ---
> Github's poor life choices should not be our problem ;-)

I thought you'd say that. :-)

> If clang-8 doesn't ship <stop_token> and doesn't work with GCC's <stop_token>, I would interpret that as you can't test <stop_token> with clang-8.

Yeah, except I'm not trying to test <stop_token>. I'm just trying to include
<condition_variable>, which includes <stop_token> and hard-errors. In fact,
what I'm _really_ trying to do is include <regex>, which includes
<memory_resource>, which includes <shared_mutex>, which includes
<condition_variable>. All roads, it seems, lead to <stop_token>.