Add logic to determine whether unnamed fmt arguments are present

[dtolnay]

This is needed for an upcoming change that disallows extra positional fmt arguments in the format string of a tuple struct or tuple variant due to ambiguity. For example in #[error("{} {0}", $N)] Variant(i32) it is ambiguous whether the {0} refers to $N vs &self.0. One would need to write a named #[error("{n} {n}", n = $N)] for the former or a numbered #[error("{n} {0}", n = $N)] for the latter, both of which have an obviously unambiguous meaning.