[red-knot] Add diagnostic for class-object access to pure instance variables #41018

Job	Run time
cargo fmt	17s
cargo build (release)	0s
Determine changes	10s
mkdocs	2m 36s
pre-commit	3m 18s
python package	1m 26s
cargo fuzz build	4m 35s
benchmarks	4m 30s
cargo build (msrv)	2m 6s
cargo clippy	1m 54s
cargo shear	22s
cargo test (linux)	2m 14s
cargo test (linux, release)	4m 35s
cargo test (wasm)	1m 44s
cargo test (windows)	6m 31s
test scripts	1m 7s
formatter instabilities and black similarity	0s
ecosystem	2m 46s
test ruff-lsp	43s
fuzz parser	0s
	40m 54s

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