[Bug libstdc++/94933] std::fill_n delegates to __builtin_memset which is not constexpr

[rhalbersma at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94933

--- Comment #2 from rhalbersma <rhalbersma at gmail dot com> ---
Sorry, my bad, but that particular overload of __fill_a1 calls
__builtin_memset. I was under the impression that this was the reason for the
missing constexpr. It would be great if simply adding _GLIBCXX20_CONSTEXPR
would fix it.