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

[egallager at gcc dot gnu.org]

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

--- Comment #6 from Eric Gallager <egallager at gcc dot gnu.org> ---
(In reply to Eric Gallager from comment #5)
> Confirmed since a bug that this blocks is confirmed.

That bug (bug 79997) is closed now btw.