[llvm] [DirectX] Add `only-section` to `llvm-objcopy` for `DXContainer` (PR #156372)

[Finn Plummer via llvm-commits]

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+## Tests that the copied DXContainer correctly retains only the RTS0 part.
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inbelic wrote:

It tests that it works as expected for the single vs many options case as some of the testing for other options does.

https://github.com/llvm/llvm-project/pull/156372