[llvm-objcopy][NFC] Refactor output target parsing
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.
Reviewers: espindola, alexshap, rupprecht, jhenderson
Reviewed By: rupprecht, jhenderson
Subscribers: jyknight, compnerd, emaste, arichardson, fedor.sergeev, jakehehrlich, MaskRay, llvm-commits
Tags: #llvm
Differential Revision: https://reviews.llvm.org/D63239
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@364254
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