[llvm] [EquivClasses] Fix signature of operator= (NFC) (PR #130140)

[Ramkumar Ramachandra via llvm-commits]

artagnon wrote:

> Can you add more description in the commit message explaining why the previous signature was incorrect? It's not obvious to me at least.



The current signature is incorrect per the C++ operator overloading spec.

https://en.cppreference.com/w/cpp/language/operators

Also added to description.

https://github.com/llvm/llvm-project/pull/130140