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_expand/src/tests.rs

@@ -4,11 +4,11 @@ use rustc_parse::{new_parser_from_source_str, parser::Parser, source_file_to_str
 use rustc_session::parse::ParseSess;
 use rustc_span::create_default_session_if_not_set_then;
 use rustc_span::source_map::{FilePathMapping, SourceMap};
-use rustc_span::{BytePos, MultiSpan, Span};
+use rustc_span::{BytePos, Span};
 
 use rustc_data_structures::sync::Lrc;
 use rustc_errors::emitter::EmitterWriter;
-use rustc_errors::{Handler, PResult};
+use rustc_errors::{Handler, MultiSpan, PResult};
 
 use std::io;
 use std::io::prelude::*;