Support meta annotation for `@ThreadSafe`

Hi, I would like to ask whether meta annotation is supported for the `@ThreadSafe` annotation.

The background is that I am trying to integrate both the Error Prone and fbinfer RacerD `@ThreadSafe` annotation (https://fbinfer.com/docs/checker-racerd) into a unified `ThreadSafe` annotation that composes both of them.

```java
@com.google.errorprone.annotations.ThreadSafe
@com.facebook.infer.annotation.ThreadSafe
public @interface ThreadSafe {}
```

This way, we do not need to have two `@ThreadSafe` annotation for the same class. However, I'm not sure if this is supported by error-prone project.

Thanks in advance. Apologies if this issue has been asked before.

No file or test is named. Start by tracing how Error Prone recognizes com.google.errorprone.annotations.ThreadSafe, then check whether annotations composing it are discovered like direct annotations. Done means the unified annotation's support is confirmed or a focused test and documentation establish the supported behavior.

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