[Bug c++/100157] Support `__type_pack_element` like Clang

[vittorio.romeo at outlook dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100157

--- Comment #6 from Vittorio Romeo <vittorio.romeo at outlook dot com> ---
Thank you, Jonathan, for looking into this. I feel like it might be worthwhile
to keep the same name as Clang for compatibility, or maybe talk to some Clang
developers and see if there can be an agreement on naming and design that works
for both compilers -- would be nice to have something that works for both GCC
and Clang in the same way.