[llvm-objcopy][ELF] Implement a mutable section visitor that updates...

[llvm-objcopy][ELF] Implement a mutable section visitor that updates size-related fields (Size, EntrySize, Align) before layout.

Summary:
Fix EntrySize, Size, and Align before doing layout calculation.

As a side cleanup, this removes a dependence on sizeof(Elf_Sym) within BinaryReader, so we can untemplatize that.

This unblocks a cleaner implementation of handling the -O<format> flag. See D53667 for a previous attempt. Actual implementation of the -O<format> flag will come in an upcoming commit, this is largely a NFC (although not _totally_ one, because alignment on binary input was actually wrong before).

Reviewers: jakehehrlich, jhenderson, alexshap, espindola

Reviewed By: jhenderson

Subscribers: emaste, arichardson, llvm-commits

Differential Revision: https://reviews.llvm.org/D56211

llvm-svn: 350336