[Bug go/83102] [8 Regression] 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 #7 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Created attachment 42679
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=42679&action=edit
Patch to make comparisons work on const objects.

Parse::Enclosing_var_comparison::operator() needs a fix too.

With this patch I can bootstrap with Go at r255051 (after that it builds
without a patch, because I relaxed the static_assert in libstdc++).