[llvm-strip] Support stripping multiple input files

Summary:
Allow strip to be called on multiple input files, which is interpreted as stripping N files in place. Using multiple input files is incompatible with -o.

To allow this, create a `DriverConfig` struct which just wraps a list of `CopyConfigs`. objcopy will only ever have a single `CopyConfig`, but strip will have N (where N >= 1) CopyConfigs.

Reviewers: alexshap, jakehehrlich

Reviewed By: alexshap, jakehehrlich

Subscribers: MaskRay, jakehehrlich, llvm-commits

Differential Revision: https://reviews.llvm.org/D51660

llvm-svn: 341464