[llvm-ar] Parse 'h' and '-h': display help and exit

Support `llvm-ar h` and `llvm-ar -h` because they may be what users try
at first. Note, operation 'h' is undocumented in GNU ar.

Reviewed By: jhenderson

Differential Revision: https://reviews.llvm.org/D67560

llvm-svn: 372088