[llvm] Revert "[Support]Look up in top-level subcommand as a fallback when looking options for a custom subcommand (PR #71975)

[Fangrui Song via llvm-commits]

MaskRay wrote:

Clicking the "revert" button gives a new PR which runs the premerge bot. For a recently landed commit that caused failures, I just run `ninja check-llvm && git push`..

https://github.com/llvm/llvm-project/pull/71975