Add DXContainer

DXIL is wrapped in a container format defined by the DirectX 11
specification. Codebases differ in calling this format either DXBC or
DXILContainer.

Since eventually we want to add support for DXBC as a target
architecture and the format is used by DXBC and DXIL, I've termed it
DXContainer here.

Most of the changes in this patch are just adding cases to switch
statements to address warnings.

Reviewed By: pete

Differential Revision: https://reviews.llvm.org/D122062