[llvm-strip] Allow only one input

Summary: Before, llvm-strip accepted a second argument but it would just be ignored.

Reviewers: alexshap, jhenderson, paulsemel

Reviewed By: alexshap

Subscribers: jakehehrlich, rupprecht, llvm-commits

Differential Revision: https://reviews.llvm.org/D51004

llvm-svn: 340229