Diagnostic cleanups

- `emitted_at` isn't used outside the crate.
- `code` and `messages` are public fields, so there's no point have
  trivial getters/setters for them.
- `suggestions` is public, so the comment about "functionality on
  `Diagnostic`" isn't needed.

compiler/rustc_errors/src/lib.rs

@@ -1422,7 +1422,7 @@ impl DiagCtxtInner {
                     &mut out,
                     "delayed span bug: {}\n{}\n",
                     bug.inner
-                        .messages()
+                        .messages
                         .iter()
                         .filter_map(|(msg, _)| msg.as_str())
                         .collect::<String>(),