ruff

eb3cb8d4 - [red-knot] Use `BitSet::union` for merging of declarations (#15451)

[red-knot] Use `BitSet::union` for merging of declarations (#15451) ## Summary In `SymbolState` merging, use `BitSet::union` instead of inserting declarations one by one. This used to be the case but was changed in https://github.com/astral-sh/ruff/pull/15019 because we had to iterate over declarations anyway. This is an alternative to https://github.com/astral-sh/ruff/pull/15419 by @MichaReiser. It's similar in performance, but a bit more declarative and less imperative.