[DirectX] Embed DXIL in LLVM Module

At the end of the codegen pipeline for DXIL we will emit the DXIL into
a global variable in the Module annotated for the "DXIL" section.

This will be used by the MCDXContainerStreamer to emit the DXIL into a
DXContainer DXIL part.

Other parts of the DXContainer will be constructed similarly by
serializing their values into GlobalVariables.

This will allow DXIL to flow into DXContainers through the normal
MCStreamer flow used in the MC layer.

Depends on D122270

Reviewed By: kuhar

Differential Revision: https://reviews.llvm.org/D125334