Implement flag to allow typechecking of untyped modules #13957

Job	Run time
Test mypyc suite with 32-bit Python	7m 1s
Test suite with py38-ubuntu, mypyc-compiled	17m 49s
Test suite with py38-windows-64	20m 12s
Test suite with py39-ubuntu	17m 7s
Test suite with py310-ubuntu	16m 43s
Test suite with py311-ubuntu, mypyc-compiled	16m 15s
Test suite with py312-ubuntu, mypyc-compiled	18m 36s
Test suite with py313-ubuntu, mypyc-compiled	18m 50s
mypyc runtime tests with py39-macos	4m 31s
Type check our own code (py38-ubuntu)	1m 55s
Type check our own code (py38-windows-64)	2m 27s
Formatting and code style with Black + ruff	57s
	2h 22m 23s

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