[lld] [ELF] Merge verdefIndex into versionId. NFC (PR #72208)

[Fangrui Song via llvm-commits]

MaskRay wrote:

Thanks for the review.

> Would it be better to move this below thunkAccessed as I think that would mean that bit could be part of the uint8_t container (I count 7 bits so far). Or is the intention to start a new container.

Unfortunately no, `thunkAccessed` is the last bit before byte offset 26.

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https://github.com/llvm/llvm-project/pull/72208