[llvm-objcopy][NFC] Use StringSaver for --keep-global-symbols

Summary: Use StringSaver/BumpPtrAlloc when parsing lines from --keep-global-symbols files. This allows us to consistently use StringRef for driver options, which avoids copying the full strings for each object copied, as well as simplifies part of D57517.

Reviewers: jhenderson, evgeny777, alexshap

Subscribers: jakehehrlich

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D57617

llvm-svn: 353068