Make struct_span_note call struct_note.

So it follows the same pattern as all the other `struct_span_*` methods.

Author: nnethercote

Diff for: compiler/rustc_errors/src/lib.rs

@@ -1179,7 +1179,7 @@ impl DiagCtxt {
         span: impl Into<MultiSpan>,
         msg: impl Into<DiagnosticMessage>,
     ) -> DiagnosticBuilder<'_, ()> {
-        DiagnosticBuilder::new(self, Note, msg).with_span(span)
+        self.struct_note(msg).with_span(span)
     }
 
     #[rustc_lint_diagnostics]