[llvm] [DirectX] Add `extract-section` to `llvm-objcopy` and implement it for `DXContainer` (PR #154804)

[James Henderson via llvm-commits]

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@@ -0,0 +1,306 @@
+## Tests that a separate DXContainer is created for the RTS0 (root signature)
+## part, specified with --extract-section specified
----------------
jh7370 wrote:

```suggestion
## part, when --extract-section is specified.
```

https://github.com/llvm/llvm-project/pull/154804