macros: support translatable labels

Extends support for generating `DiagnosticMessage::FluentIdentifier`
messages from `SessionDiagnostic` derive to `#[label]`.

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

compiler/rustc_errors/src/diagnostic.rs

@@ -277,7 +277,7 @@ impl Diagnostic {
     ///
     /// This span is *not* considered a ["primary span"][`MultiSpan`]; only
     /// the `Span` supplied when creating the diagnostic is primary.
-    pub fn span_label<T: Into<String>>(&mut self, span: Span, label: T) -> &mut Self {
+    pub fn span_label(&mut self, span: Span, label: impl Into<DiagnosticMessage>) -> &mut Self {
         self.span.push_span_label(span, label.into());
         self
     }