Fix typechecking for async generators #12760

Job	Run time
Test mypyc suite with 32-bit Python	7m 4s
Test suite with py38-ubuntu, mypyc-compiled	17m 39s
Test suite with py38-windows-64	19m 49s
Test suite with py39-ubuntu	16m 10s
Test suite with py310-ubuntu	15m 17s
Test suite with py311-ubuntu, mypyc-compiled	15m 39s
Test suite with py312-ubuntu, mypyc-compiled	17m 47s
mypyc runtime tests with py39-macos	4m 34s
mypyc runtime tests with py38-debug-build-ubuntu	4m 23s
Type check our own code (py38-ubuntu)	1m 44s
Type check our own code (py38-windows-64)	2m 21s
Formatting and code style with Black + ruff	36s
	2h 3m 3s

Provide feedback