`jsii` used to fail when trying to generate deprecation warnings and
given a statement like this:

```ts
export function propertyInjectionDecorator<T extends Constructor>(ctr: T) {
  // Important for the bug: the anonymous class extends something, *and*
  // declares a method.
  return class extends ctr {
    public someMethod(): string {
      return 'abc';
    }
  };
}
```

The reason is that during generation of deprecation warnings we iterate
over all classes and methods (both exported and unexported), and
generate a symbol identifier for every method; but this class is
anonymous
so it doesn't have a symbol, and generating the identifier fails with
the error:

```
TypeError: Cannot read properties of undefined (reading 'flags')
    at symbolIdentifier (.../jsii/lib/common/symbol-id.js:40:17)
```

Handle this by handling the case where we don't have a symbol
and returning `undefined`.

Fixes aws/aws-cdk#33213

---

By submitting this pull request, I confirm that my contribution is made
under the terms of the [Apache 2.0 license].

[Apache 2.0 license]: https://www.apache.org/licenses/LICENSE-2.0

---------

Co-authored-by: Eli Polonsky <Eli.polonsky@gmail.com>
(cherry picked from commit dbd4506)