[llvm-objcopy] - Many minor NFC changes to cleanup/improve the code in ELF/Object.cpp.

The code in ELF/Object.cpp is sometimes a bit hard to read because of
lots of auto used everywhere. The main intention of this patch is
to replace them with the real type for places where it is not obvious.
Also it cleanups few places.

It is NFC change, but I want to be sure that there is no objections to do that since it
is massive.

DIfferential revision: https://reviews.llvm.org/D62260

llvm-svn: 361466