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

[dmalcolm at gcc dot gnu.org]

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

--- Comment #4 from David Malcolm <dmalcolm at gcc dot gnu.org> ---
Patch looks reasonable, and I can approve it.

CC me on it when you post it to gcc-patches.