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

[James Henderson via llvm-commits]

================
@@ -19,3 +19,22 @@ Parts:
   - Name:            FKE0
     Size:            8
 ...
+
+## We can't extract a part that is specified incorrectly.
+# RUN: yaml2obj %s --docnum=2 -o %t2
+# RUN: not llvm-objcopy %t2 --extract-section=FKE0,%t.fke0.out 2>&1 | FileCheck %s -DFILE=%t2 --check-prefix=ERROR2
+
+# ERROR2: error: bad format for --extract-section, expected section=file
+
+--- !dxcontainer
+Header:
+  Hash:            [ 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0,
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jh7370 wrote:

Nit: here and elsewhere, we tend to remove excessive whitespace between the key and value.

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