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

[Nikita Popov via llvm-commits]

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

LGTM. The current operator is valid (I think the return value can be arbitrary), but it's certainly unusual.

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