[llvm-ar] Implement the V modifier as an alias for --version

Summary: Also update the help modifier (h) so that it works as a modifier and not just as a standalone `h`. For example, `llvm-ar h` prints the help message, but `llvm-ar xh` currently prints `unknown option h`.

Reviewers: MaskRay, gbreynoo

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D69007

llvm-svn: 375028