cpp: how to use macro argument as dataflow sink

I would like to do something like this:
```codeql
predicate isSink(DataFlow::Node sink) {
exists(MacroInvocation m |
sink.asExpr() = m.getArgument(0)
and m.getTarget().getName() = "my_cool_macro"
)
}
```
But `getArgument` is not defined, and `getExpandedArgument` returns a string. Doing `sink.asExpr().toString() = m.getExpandedArgument(0)` returns zero matches.

How do I use macros in dataflow?

Start with the CodeQL predicate in the issue and read the mentioned MacroInvocation, getArgument, and getExpandedArgument entry points. Determine how macro arguments can be represented as dataflow nodes, or whether this is unsupported, and document a working example or the limitation so the question has a definitive answer.

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