[Bug libstdc++/99058] Consider adding a note about std::optional ABI break to the C++17 status table

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99058

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
C++17 support isn't stable until GCC 9 so there is no guarantee of
compatibility between 7 and 8 or 8 and 9. That applies to the entire library
(and language features) not just std::optional.