[llvm-ar] Support multiple dashed options

This allows syntax like:
$ llvm-ar -c -r -u file.a file.o

This is in addition to the other formats that are already supported:
$ llvm-ar cru file.a file.o
$ llvm-ar -cru file.a file.o

Patch by Tom Anderson!

Differential Revision: https://reviews.llvm.org/D44452

llvm-svn: 328716