_Noreturn and draft C23

[Paul Eggert]

That reminds me, we're in a sticky situation about _Noreturn for another 
reason.

Draft C23 requires [[noreturn]] before "extern" and states that 
_Noreturn is obsolescent. It's plausible that a future C version will 
drop the requirement to support the _Noreturn keyword. If that happens, 
"#define _Noreturn [[noreturn]]" won't be a valid workaround unless 
people write "_Noreturn extern" instead of "extern _Noreturn", and 
similarly for "_Noreturn static" etc.

Should we change Gnulib code and put _Noreturn first in its declarations 
now, proactively? This might help future-proof the code, and get people 
used to putting directives first. (But of course it would be annoying to 
churn in this way.)