[Bug bootstrap/83102] go bootstrap error in ast-dump.cc due to __is_invocable failure

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83102

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #2)
> This fails, because the comparison object in the set is const, and so it's

s/it's/its/


The requirements in the standard were clarified by https://wg21.link/lwg2542 so
I suppose strictly-speaking we should only reject it in C++17 mode, but our
usual policy is to backport DRs to all modes where relevant (and we never
supported the example in the DR, so the static_assert is just enforcing what
was already the case in our implementation).