5478: Replace existing visibility modifier in fix_visibility r=matklad a=TimoFreiberg
Fixes#4636
I would have liked to do something about the `// FIXME: this really should be a fix for diagnostic, rather than an assist.`, but that would take a while and there's no reason not to fix this immediately.
Co-authored-by: Timo Freiberg <timo.freiberg@gmail.com>