Add verbose option to rustdoc in order to fix problem with --version

Patch for #20340. `rustdoc --version` panics because it uses `rustc_driver::version`, which in turn checks the `verbose` flag, which was not defined for rustdoc.  In this patch I have added a verbose flag to rustdoc, because I think it should be useful for other things besides --version. 

Another possible fix would be to check if a verbose option was defined in `rustc_driver` or add an extra `version` function for rustdoc.