[Bug c++/88336] Implement P0595R2, C++20 std::is_constant_evaluated (compiler magic library tool).

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88336

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
__builtin_constant_evaluated is already implemented, it just needs to be
provided with the name std::is_constant_evaluated() (and possibly adjusted for
the changes in the R2 paper).