[DirectX] Add `only-section` to `llvm-objcopy` for `DXContainer` (#156372)
This pr implements the `only-section` option for a `DXContainer` object
in `llvm-objcopy`.
It extends the base `removeParts` of the minimal object representation
of a `DXContainerObject`.
This a pre-requisite step to implement
https://github.com/llvm/llvm-project/issues/150286 as a way to strip all
other information away.