[Bug c++/79452] Provide builtin to detect compile-time execution

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79452

--- Comment #12 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Well this request is satisfied by std::is_constant_evaluated() in C++20, which
is what __builtin_is_constant_evaluated() exists for. PR 100974 adds a newer
C++23 language feature that makes std::is_constant_evaluated() unnecessary.
This is still resolved one easy or another though.