[Bug preprocessor/77810] provide format_inform_at_substring to go with format_warning_at_substring

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77810

--- Comment #3 from Martin Sebor <msebor at gcc dot gnu.org> ---
I haven't posted the patch for review.  IIRC, I put it together in hopes of
enhancing the -Wformat-length notes to refer to the arguments of the directives
and offering fixit hints showing how to resolve some of the warnings.  The
motivation for this was bug 77690 (and similar cases) where the solution is to
add precision to a %s directive to cap the number of bytes it outputs.  The
general enhancement of adding fixit notes to the warning is being tracked in
bug 77831.  I'd be interested in your ideas for how they could be put to use
beyond the %s directive and where (this might perhaps be worth tackling in
conjunction with bug 77696).

If there's still time to work on this in stage 3 I'll dust off the patch and
post it for review.