[ObjectYAML][DX] Support yaml2dxcontainer

This patch adds a the first bits of support for a yaml representation
of dxcontainer files.

Since the YAML representation's primary purpose is testing
infrastructure, the yaml representation supports both verbose and a
more friendly format by making computable sizes and offsets optional.
If provided they are validated to be correct, otherwise they are
computed on the fly during emission.

As I expand the format I'll be able to make more size fields optional,
and I will continue to make the format easier to work with.

Depends on D124804

Reviewed By: lhames

Differential Revision: https://reviews.llvm.org/D124944