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

[Alexandre Ganea via llvm-commits]

aganea wrote:

> C++20 will automatically generate an operator== with reversed operand order, which is ambiguous with the written operator== when one argument is marked const and the other isn't.

You mean when building LLD with `cmake ... -DCMAKE_CXX_STANDARD=20`?

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