[Bug c++/86974] Support Clang's require_constant_initialization attribute

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86974

--- Comment #3 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
I think this is now "superseded" by PR91360, C++20 constinit.