507: Fix handling of attributes in positional field lists r=matklad a=DJMcNab
First reported by @max-frai. This allows us to properly handle crates using e.g. `#[Derive(fail)]` with `#[fail(cause)]`, among other cases.
Co-authored-by: DJMcNab <36049421+djmcnab@users.noreply.github.com>