[DirectX] Add DirectX target object writer

This is the last piece to bring together writing DXContainer files
containing DXIL through the DirectX backend.

While this change only has one test, all of the tests under
llvm/test/tools/dxil-dis also exercise this code. With this change the
output object file type for the dxil target is now DXContainer. Each of
the existing tests will generate DXContainer files, and the dxil-dis
tests additionally verify that the DXContainers generated are
well-formed and can be parsed by the DirectXShaderCompiler tools.

Depends on D127153 and D127165

Differential Revision: https://reviews.llvm.org/D127166