[llvm] [ObjectYAML][NFC] Finalize COFF symbol table offset in writeCOFF (PR #212671)

[James Henderson via llvm-commits]

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jh7370 wrote:

I'd store the BaseOffset input parameter in a constant that can be used in the test checks. It helps show that the 4 here is directly tied to the `getOffset` values.

https://github.com/llvm/llvm-project/pull/212671