[llvm] [mlir][Bazel] Add a comment and move deps list into an extra line. (PR #93453)

[Adrian Kuegel via llvm-commits]

akuegel wrote:

Some additional information: What we want to do is to add an extra block of optional dependencies like this:

```
        # begin-marker
        if_cuda_available([
            "//third_party/gpus/cuda:cuda_path",
        ]) +
        # end-marker
```

We have a conversion script that strips this off when converting to open source BUILD.bazel, and it needs to reinsert this when transforming back. Without this change, our internal BUILD file would have a different level of indentation which causes an error. If we also have a comment in the BUILD.bazel file and start the deps list on the next line, it will have the same level of indentation.

https://github.com/llvm/llvm-project/pull/93453