bug#25987: 25.2; support gcc fixit notes

[Eli Zaretskii]

> From: David Malcolm <dmalcolm@redhat.com>
> Cc: 25987@debbugs.gnu.org
> Date: Tue, 20 Oct 2020 10:52:05 -0400
> 
> One possible issue: in the final diagnostic, there's a fix-it hint with
> non-ASCII replacement text, replacing "two_pi" with "two_π" (where the
> final char in the latter is GREEK SMALL LETTER PI, U+03C0)
> 
> This replacement currently expressed as encoded bytes i.e:
> 
> fix-it:"demo.c":{51:10-51:16}:"two_\317\200"
> 
> where \317\200 is the octal-escaped representation of the two bytes of
> the UTF-8 encoding of the character.
> 
> Is this going to work for Emacs?

You mean, GCC doesn't actually emit the UTF-8 encoding of π, it emits
its ASCII-fied representation?  We'd need to decode that, but is that
really justified?  Why not emit UTF-8?