[lld] Mark operator== const to avoid ambiguity in C++20. (PR #68119)

[Alexandre Ganea via llvm-commits]

https://github.com/aganea approved this pull request.

I reproduced the issue, thanks for the fix! LGTM. I can merge the change if there are no further objections from @mstorsjo 

https://github.com/llvm/llvm-project/pull/68119