[llvm-objcopy][NFC] Refactor output target parsing v2

Summary:
Use an enum instead of string to hold the output file format in Config.InputFormat and Config.OutputFormat. It's essential to support other output file formats other than ELF.

This patch originally has been submitted as D63239. However, there was an use-of-uninitialized-value bug and reverted in r364379 (git commit 4ee933c7).

This patch includes the fix for the bug by setting Config.InputFormat/Config.OutputFormat in parseStripOptions.

Reviewers: espindola, alexshap, rupprecht, jhenderson

Reviewed By: jhenderson

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D64170

llvm-svn: 365173