Both diagnostic and subdiagnostic derives were missing the ability to
add warnings to diagnostics - this is made more difficult by the `warn`
attribute already existing, so this name being unavailable for the
derives to use. `#[warn_]` is used instead, which requires
special-casing so that `{span_,}warn` is called instead of
`{span_,}warn_`.
Signed-off-by: David Wood <david.wood@huawei.com>
81cf2294b4