[NFC] Refactor DXContainer to support more parts

This patch refactors some of the DXContainer Object and YAML code to
make it easier to add more part parsing.

DXContainer has a whole bunch of constant values, so I've added a
DXContainerConstants.def file which will grow with constant
definitions, but starts with just part identifiers. I've also added a
utility to parse the part magic string into an enum, and converted the
code to use that utility and the enum instead of the part literal
string.

Reviewed By: lhames

Differential Revision: https://reviews.llvm.org/D133980