[DirectX] Fix DXIL part header version encoding

[llvm-beanz]

Today we are encoding the DXIL version in the DXIL part's program header backwards. Compiling a shader targeting lib_6_8 with DXC then disassembling it with obj2yaml prints:

      DXILMajorVersion: 8
      DXILMinorVersion: 1

This probably means we're crossing the ordering of the fields somewhere in our handling.

We also need to set the DXIL version from the triple sub-architecture. Right now during DXIL header generation we are not setting the version. See here where we set the shader model version but we don't set the DXIL version.

Acceptance Criteria

• Updated obj2yaml tests verifying correct reproduction of major and minor versions.
• Updated llc -> obj2yaml testing verifying correct code generation from llc.