Implement flag to allow typechecking of untyped modules #13107

Job	Run time
Test suite with Python nightly	14m 30s
Test mypyc suite with 32-bit Python	6m 46s
Test suite with py38-ubuntu, mypyc-compiled	17m 54s
Test suite with py38-windows-64	20m 12s
Test suite with py39-ubuntu	16m 25s
Test suite with py310-ubuntu	16m 16s
Test suite with py311-ubuntu, mypyc-compiled	16m 9s
Test suite with py312-ubuntu, mypyc-compiled	17m 45s
mypyc runtime tests with py39-macos	7m 15s
mypyc runtime tests with py38-debug-build-ubuntu	4m 26s
Type check our own code (py38-ubuntu)	1m 50s
Type check our own code (py38-windows-64)	2m 11s
Formatting and code style with Black + ruff	38s
	2h 22m 17s

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