Don’t leak unreachability from lambda body to surrounding scope #12468

Job	Run time
Test mypyc suite with 32-bit Python	7m 25s
Test suite with py38-ubuntu, mypyc-compiled	17m 45s
Test suite with py38-windows-64	20m 18s
Test suite with py39-ubuntu	16m 35s
Test suite with py310-ubuntu	15m 46s
Test suite with py311-ubuntu, mypyc-compiled	15m 21s
Test suite with py312-ubuntu, mypyc-compiled	17m 27s
mypyc runtime tests with py39-macos	4m 59s
mypyc runtime tests with py38-debug-build-ubuntu	4m 29s
Type check our own code (py38-ubuntu)	1m 51s
Type check our own code (py38-windows-64)	2m 46s
Formatting and code style with Black + ruff	36s
	2h 5m 18s

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