c45f29595d
`MultiSpan` contains labels, which are more complicated with the introduction of diagnostic translation and will use types from `rustc_errors` - however, `rustc_errors` depends on `rustc_span` so `rustc_span` cannot use types like `DiagnosticMessage` without dependency cycles. Introduce a new `rustc_error_messages` crate that can contain `DiagnosticMessage` and `MultiSpan`. Signed-off-by: David Wood <david.wood@huawei.com>