This allows us to avoid synthesizing tokens in `prepend_attr`, since we
have the original tokens available.
We still need to synthesize tokens when expanding `cfg_attr`,
but this is an unavoidable consequence of the syntax of `cfg_attr` -
the user does not supply the `#` and `[]` tokens that a `cfg_attr`
expands to.
We currently only attach tokens when parsing a `:stmt` matcher for a
`macro_rules!` macro. Proc-macro attributes on statements are still
unstable, and need additional work.