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_codegen_llvm/src/errors.rs

@@ -102,7 +102,7 @@ pub(crate) struct ParseTargetMachineConfig<'a>(pub LlvmError<'a>);
 impl<G: EmissionGuarantee> IntoDiagnostic<'_, G> for ParseTargetMachineConfig<'_> {
     fn into_diagnostic(self, dcx: &'_ DiagCtxt, level: Level) -> DiagnosticBuilder<'_, G> {
         let diag: DiagnosticBuilder<'_, G> = self.0.into_diagnostic(dcx, level);
-        let (message, _) = diag.messages().first().expect("`LlvmError` with no message");
+        let (message, _) = diag.messages.first().expect("`LlvmError` with no message");
         let message = dcx.eagerly_translate_to_string(message.clone(), diag.args());
 
         DiagnosticBuilder::new(dcx, level, fluent::codegen_llvm_parse_target_machine_config)