[mlir][async] Fix missing dialect dependency in async-to-async-runtime

[mlir][async] Fix missing dialect dependency in async-to-async-runtime

The specific case I hit this was when lowering an `async.execute` that's
not inside a `func.func` op, e.g.:
```
llvm.func @foo() {
  %token = async.execute {
    async.yield
  }
  llvm.return
}
```

Reviewed By: bondhugula

Differential Revision: https://reviews.llvm.org/D135742