[llvm-objcopy][ELF] Add OriginalType & OriginalFlags

`llvm::objcopy::elf::*Section::classof` matches Type and Flags, yet Type
and Flags are mutable (by setSectionFlagsAndTypes and upcoming
--only-keep-debug feature). Add OriginalType & OriginalFlags to be used
in classof, to prevent classof results from changing.

Reviewed By: jakehehrlich, jhenderson, alexshap

Differential Revision: https://reviews.llvm.org/D69739