ELF: Use StringRef instead of std::string.
ELF: Use StringRef instead of std::string.

All MemoryBuffers for archive files are guaranteed to exist as long
as their children are used in the linker. So we don't need to copy
strings here. Thanks to Sean Silva for pointing this out.

git-svn-id: f6089bf0e6284f307027cef4f64114ee9ebb0424@259554 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/lld,llvm-mirror/lld