bug#25987: 25.2; support gcc fixit notes

[Eli Zaretskii]

> From: David Malcolm <dmalcolm@redhat.com>
> Cc: 25987@debbugs.gnu.org
> Date: Wed, 14 Oct 2020 18:43:33 -0400
> 
> In his email, Andrea suggests outputting to a file.  How would 
> that work?  It strikes me as making it difficult to associate the
> output from stderr with that to the file, or would the output to the
> file need to replace that from stderr (in which case what about lines
> of output from "make"? or other build system messages, etc).

I'm not sure how a separate file comes into this.  Aren't we talking
about the "normal" GCC diagnostic output, just augmented by hints?
That has the advantage that it is also human-readable, and could help
the user make changes other than accepting the hints.