[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 #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Changing the _Stop_state_ref default constructor to:

    _Stop_state_ref() noexcept {}

allows it to compile. It shouldn't be needed, but I suppose since the
constructor is already non-trivial (due to the default member initializer that
is the cause of the error) we could do that.