AbstractMessager

[Ron Shapiro]

In a number of processors that I've implemented, I've wanted to wrap a
Messager instance to track if I've printed a diagnostic with an ERROR kind
so I can know to abort any generation if the inputs are invalid.

There are a number of ways to do this, but what always feels like would be
most natural is to be able to create a ForwardingMessager of some sort. I
think the easiest way to do this is if there were already an
AbstractMessager class, so I wanted to propose adding it to the JSR 269
APIs.

The implementation would be rather simple, just telescoping the calls with
`null` arguments, just like JavacMessager works.

@Override
public void printMessage(Diagnostic.Kind kind, CharSequence msg) {
    printMessage(kind, msg, null);
}

@Override
public void printMessage(Diagnostic.Kind kind, CharSequence msg, Element e)
{
    printMessage(kind, msg, e, null);
}

// etc

Alternatively, the methods could become default methods, which is probably
the easiest change and doesn't introduce a new type.

Does something like this make sense?
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