errors: introduce DiagnosticMessage

Introduce a `DiagnosticMessage` type that will enable diagnostic
messages to be simple strings or Fluent identifiers.
`DiagnosticMessage` is now used in the implementation of the standard
`DiagnosticBuilder` APIs.

Signed-off-by: David Wood <david.wood@huawei.com>

compiler/rustc_parse/src/parser/item.rs

@@ -1010,7 +1010,8 @@ impl<'a> Parser<'a> {
                 let current_qual_sp = self.prev_token.span;
                 let current_qual_sp = current_qual_sp.to(sp_start);
                 if let Ok(current_qual) = self.span_to_snippet(current_qual_sp) {
-                    if err.message() == "expected `{`, found keyword `unsafe`" {
+                    // FIXME(davidtwco): avoid depending on the error message text
+                    if err.message().as_str() == "expected `{`, found keyword `unsafe`" {
                         let invalid_qual_sp = self.token.uninterpolated_span();
                         let invalid_qual = self.span_to_snippet(invalid_qual_sp).unwrap();