span: move MultiSpan

`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>

Author: davidtwco

src/parse/session.rs

@@ -313,7 +313,8 @@ mod tests {
         use super::*;
         use crate::config::IgnoreList;
         use crate::utils::mk_sp;
-        use rustc_span::{FileName as SourceMapFileName, MultiSpan, RealFileName};
+        use rustc_errors::MultiSpan;
+        use rustc_span::{FileName as SourceMapFileName, RealFileName};
         use std::path::PathBuf;
         use std::sync::atomic::AtomicU32;