Added "-help" option to Options.td
Added also:
* Added "OPT_HELP" case in the main TestOptions.cpp switch. It uses the llvm options help functionality to provide up-to-date help
* Additional "Use -help for assistance" at the end of the error message that appears when calling an unknown option
Added their following tests, respectively::
* usage.swift, and
* wrong_arguments.swift
Extra: FIXME: in TestOptions::printHelp, suggesting a possible expansion for the printHelp option (details in file)
