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>

compiler/rustc_middle/src/util/bug.rs

@@ -1,7 +1,8 @@
 // These functions are used by macro expansion for bug! and span_bug!
 
 use crate::ty::{tls, TyCtxt};
-use rustc_span::{MultiSpan, Span};
+use rustc_errors::MultiSpan;
+use rustc_span::Span;
 use std::fmt;
 use std::panic::{panic_any, Location};