[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 #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
It works fine in Clang 9.0.0 though. We've never really set a hard limit on how
many old versions of Clang we support, but given that 9 and 10 (and trunk)
work, I'm inclined to think that three versions seems enough for new C++20
features.