[CommandLine] Better report unknown subcommands (#74811)

The patch improves the reporting for the first option in the command
line when it looks like a subcommand name but does not match any
defined.

Before the patch:
```
> prog baz
prog: Unknown command line argument 'baz'.  Try: 'prog --help'
```

With the patch:
```
> prog baz
prog: Unknown subcommand 'baz'.  Try: 'prog --help'
prog: Did you mean 'bar'?
```