[Object][DX] Initial DXContainer parsing support

This patch begins adding DXContainer parsing support to libObject.
Following the pattern used by ELFFile my goal here is to write a
standalone DXContainer parser and later write an adapter interface to
support a subset of the ObjectFile interfaces so that we can add
limited objdump support. I will also be adding ObjectYAML support to
help drive testing of the object tools and MC-level object writers as
those come together.

DXContainer is a slightly odd format. It is arranged in "parts" that
are semantically similar to sections, but it doesn't support symbol
listing.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D124643