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

[redi at gcc dot gnu.org via Gcc-bugs]

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

--- Comment #7 from Jonathan Wakely <redi at gcc dot gnu.org> ---
g:f5d9bc8ae81abe46640477bc9e655aa093947f5f removed the dependency from
<shared_mutex> on <condition_variable> for most targets.