Rework SessionSubdiagnostic derive to support multipart_suggestion

compiler/rustc_macros/src/lib.rs

@@ -171,8 +171,13 @@ decl_derive!(
         suggestion_short,
         suggestion_hidden,
         suggestion_verbose,
+        multipart_suggestion,
+        multipart_suggestion_short,
+        multipart_suggestion_hidden,
+        multipart_suggestion_verbose,
         // field attributes
         skip_arg,
         primary_span,
+        suggestion_part,
         applicability)] => diagnostics::session_subdiagnostic_derive
 );