[Bug c++/97573] Implement C++20 [depr.arith.conv.enum]

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97573

--- Comment #1 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Likewise, [depr.array.comp] should be implemented too.