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

[Samira Bazuzi via llvm-commits]

bazuzi wrote:

@mstorsjo I don't have write access, so can't specifically add reviewers, but I see that you reviewed the last change touching this file. Would you be able to take a look at this small change?

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