macros: subdiagnostic derive

Add a new derive, `#[derive(SessionSubdiagnostic)]`, which enables
deriving structs for labels, notes, helps and suggestions.

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

compiler/rustc_errors/src/diagnostic_builder.rs

@@ -530,6 +530,11 @@ impl<'a, G: EmissionGuarantee> DiagnosticBuilder<'a, G> {
         name: impl Into<Cow<'static, str>>,
         arg: DiagnosticArgValue<'static>,
     ) -> &mut Self);
+
+    forward!(pub fn subdiagnostic(
+        &mut self,
+        subdiagnostic: impl crate::AddSubdiagnostic
+    ) -> &mut Self);
 }
 
 impl<G: EmissionGuarantee> Debug for DiagnosticBuilder<'_, G> {