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

[redi at gcc dot gnu.org]

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

--- Comment #8 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Vittorio Romeo from comment #6)
> worthwhile to keep the same name as Clang for compatibility,

No, that's not an option. Clang's is a built-in template, GCC's can't be (it
would require considerable internal reworking to support that).

That's also why we have __integer_pack(N)... instead of __make_integer_seq<>.

Since GCC's built-in has to use different syntax, it would be a disaster to use
the same name.

#if __has_builtin(__type_pack_element)
// now what? is it a template or a function?
#endif