[Option] Support MultiArg in --help

Currently, if you use a MultiArg<> option, then printing out the help/usage
message will cause an assert.  This fixes getOptionHelpName() to work with
MultiArg Options.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215770 91177308-0d34-0410-b5e6-96231b3b80d8