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

[redi at gcc dot gnu.org]

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

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Github's poor life choices should not be our problem ;-)

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.