This fixes an incorrect suggestion from the `useless_attribute` lint
when using `cfg_attr`.
Additionally, it will not show a suggestion anymore, if the attribute
begins on a previous line, because it is much harder to construct the
span of multi-line `cfg_attr` attributes as they don't appear in the AST.
To fix it completely, one would have to parse upwards into the file,
and find the beginning of the `cfg_attr` attribute.
