Reland "[llvm-objcopy] Add support for nested and overlapping segments"

I didn't initialize a pointer to be nullptr that I needed to.

This change adds support for nested and even overlapping segments. This means
that PT_PHDR, PT_GNU_RELRO, PT_TLS, and PT_DYNAMIC can be supported properly.

Differential Revision: https://reviews.llvm.org/D36558

llvm-svn: 313682