[yaml2obj] Refactor command line parsing

* Hide unrelated options.
* Add "OVERVIEW: " to yaml2obj -h/--help.
* Place options under a yaml2obj category.
* Disallow -docnum. Currently -docnum is the only yaml2obj specific long option that is affected.
* Specify `cl::init("-")` and `cl::Prefix` for OutputFilename. The
  latter allows `-ofile`

Reviewed By: grimar, jhenderson

Differential Revision: https://reviews.llvm.org/D73982