[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 #1 from Martin Sebor <msebor at gcc dot gnu.org> ---
Created attachment 39727
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=39727&action=edit
Prototype patch.

I tested the attached proof-of-concept patch by integrating it into the
gimple-ssa-sprintf pass.