bug#25987: 25.2; support gcc fixit notes

[David Malcolm]

[I'm not familiar with this bug-tracking system; let's see if this
works]

I'm the author of the -fdiagnostics-parseable-fixits feature in GCC; I
maintain the GCC diagnostics subsystem.

As Tom(?) noted, I made -fdiagnostics-parseable-fixits report in bytes,
partly because that's the status quo for all of GCC's "column"
reporting, and partly because that's what clang implemented for this. 
It appears that Emacs can already decode from byte-offsets in a line
back to columns, so this seems like a compatibility detail in case we
fix everything to use real column numbers.

I'm also an Emacs user (though not a Emacs developer, sorry; my Lisp is
practically non-existent) - I just wondered what the status of this is.
I'm keen to be able to use it (GCC will get better faster!), even if
it's just a new command in compilation mode to go ahead and apply a
fix-it hint for the current diagnostic, as seen by next-error, if it
has one, maybe prettifying the printing of such lines, with the user
having to manually supply -fdiagnostics-parseable-fixits somehow.

FWIW GCC 8 is gaining lots more fix-it hints (including ones that add
newlines, for missing "#include" suggestions); see:

https://developers.redhat.com/blog/2018/03/15/gcc-8-usability-improvements/

for a blog post I wrote about it.

Thanks; hope this is constructive.
Dave