[llvm-strip] Error when using stdin twice

Summary: Implements bug [[ https://bugs.llvm.org/show_bug.cgi?id=42204 | 42204 ]]. llvm-strip now warns when the same input file is used more than once, and errors when stdin is used more than once.

Reviewers: jhenderson, rupprecht, espindola, alexshap

Reviewed By: jhenderson, rupprecht

Subscribers: emaste, arichardson, jakehehrlich, MaskRay, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D63122

llvm-svn: 363638