Auto merge of #49349 - Zoxc:sync-errors, r=michaelwoerister

Auto merge of #49349 - Zoxc:sync-errors, r=michaelwoerister

Make Handler more thread-safe

The use of `code_emitted` to suppress extended explanations is not thread safe. I'm not sure why we keep the documentation for errors outside `diagnostics.rs` anyway. It would be better to add a `teach` method to `DiagnosticsBuilder`, so instead of:
```
if self.tcx.sess.teach(&err.get_code().unwrap()) {
    err.note("...");
}
```
we'd use `err.teach("...")`

cc @estebank

r? @michaelwoerister