[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 #7 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to m.cencora from comment #5)
> Yeah, __is_same builtin beats custom unique-id comparisons, but it is
> available only since gcc-10 so unavailable for me.

GCC has had __is_same_as since 6.1 though. Much later, __is_same was added as
another spelling for it (PR 92271).