[llvm] [CommandLine][NFCI] Do not add 'All' to 'RegisteredSubCommands' (PR #77722)

[Fangrui Song via llvm-commits]

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

Thanks for the simplification. The main trick appears to be `forEachSubCommand`

https://github.com/llvm/llvm-project/pull/77722