[llvm-objcopy] Rename variable names "Section" to "Sec". NFC

"Section" can refer to the type llvm::objcopy::elf::Section or the
variable name. Rename it to "Sec" for clarity. "Sec" is already used a
lot, so this change improves consistency as well.

Also change `auto` to `const SectionBase` for readability.

Reviewed By: grimar

Differential Revision: https://reviews.llvm.org/D67143

llvm-svn: 370852