OptTable SubCommand is not usable with positional command-line arguments

https://github.com/llvm/llvm-project/pull/155026 adds the support for subcommand for llvm::OptTable.

However, the new addition cannot be used when the tool needs to have additional positional arguments. The current implementation will give up when there are more than 1 positional arguments.

Ideally, the desired behavior is that the first matching subcommand is treated as real sub-command, while the rest of the positional arguments are treated as INPUT.