[Bug c++/106784] Add __is_convertible built-in

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106784

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
To be clear, I'd like __is_nothrow_convertible too. But if I only get
__is_convertible first, that would still be great.