[llvm] [llvm-objdump] Implement decoding auxiliary header for xcoff with llvm-objdump --private-headers (PR #105682)

[James Henderson via llvm-commits]

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+## Test that `the llvm-objdump --private-headers` print out the auxiliary header of XCOFF object file.
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jh7370 wrote:

According to the pre-merge checks, this test is failing on Linux. Please review and fix.

I've also previously said that I'd prefer CHECK lines to be immediately after the corresponding test case, which you haven't addressed here.

https://github.com/llvm/llvm-project/pull/105682