[Bug libstdc++/89022] Implement P0202R3 - C++20 Constexpr Modifiers to Functions in and Headers.

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89022

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Oh, and I removed __cpp_lib_constexpr from <version> today.