bug#25987: 25.2; support gcc fixit notes

[Tom Tromey]

Eli> Is there perhaps some "prior art" for this in other IDEs out there?

Tom> It's a good question but I don't know the answer.

David Malcolm (who wrote the gcc feature) pointed me at the Eclipse bug
for this:

https://bugs.eclipse.org/bugs/show_bug.cgi?id=497670

It sounds like they're going to integrate it into their "quick fix"
feature, which is an Eclipse thing.

They also discuss the need to attach the fixit to an earlier error.

Tom