[clang analysis][thread-safety] Handle return-by-reference... (#67776)

...of guarded variables, when the function is not marked as requiring
locks:

```
class Return {
  Mutex mu;
  Foo foo GUARDED_BY(mu);

  Foo &returns_ref_locked() {
    MutexLock lock(&mu);
    return foo;  // BAD
  }

  Foo &returns_ref_locks_required() SHARED_LOCKS_REQUIRED(mu) {
    return foo;  // OK
  }
};
```

Review on Phabricator: https://reviews.llvm.org/D153131