[red-knot] Use `BitSet::union` for merging of declarations (#15451) #38726

Job	Run time
cargo fmt	17s
Determine changes	10s
mkdocs	2m 32s
cargo build (release)	7m 2s
pre-commit	3m 5s
python package	1m 24s
benchmarks	5m 0s
cargo clippy	1m 58s
cargo fuzz build	4m 29s
cargo build (msrv)	4m 12s
cargo shear	23s
cargo test (linux, release)	3m 43s
cargo test (linux)	1m 46s
cargo test (wasm)	1m 45s
cargo test (windows)	4m 19s
formatter instabilities and black similarity	3m 16s
test scripts	1m 5s
test ruff-lsp	37s
ecosystem	0s
fuzz parser	0s
	47m 3s

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