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

[Alexander Richardson via llvm-commits]

arichardson 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.
> 
> (ccls-member-hierarchy) [ ![a](https://user-images.githubusercontent.com/416322/282880287-cebac46e-5058-44b6-932f-423b0ac8c726.jpg) ](url)

Ah yes, I failed to scroll up far enough!

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