[llvm] [Support]Look up in top-level subcommand as a fallback when looking options for a custom subcommand. (PR #71776)

[Snehasish Kumar via llvm-commits]

https://github.com/snehasish approved this pull request.

lgtm though please wait a bit for feedback from @MaskRay 

https://github.com/llvm/llvm-project/pull/71776