[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 #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Because otherwise it's impossible to use a const set:

const Import_init_set s;
auto it = s.lower_bound(0);

This fails, because the comparison object in the set is const, and so it's
operator() isn't usable. The Go code apparently never calls const member
functions on those sets.