Improve pretty printer for comma-separated VarDecls and FieldDecls

```int a __attribute__((annotate("a"))), b __attribute__((annotate("b")));```

In the above, we'll print this roughly as:
```
int a __attribute__((annotate("a")));
int b __attribute__((annotate("b")))
```

This affects the algorithm in `PrintedTokenRange::AlignTokens`.
