[llvm-objdump] Allow short options without arguments to be grouped

Summary:

https://bugs.llvm.org/show_bug.cgi?id=31679

Reviewers: kristina, jhenderson, grimar, jakehehrlich, rupprecht

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D57904

llvm-svn: 353998