stubgen: Preserve simple defaults in function signatures #11678

Job	Run time
Test mypyc suite with 32-bit Python	8m 29s
Test suite with py38-ubuntu, mypyc-compiled	19m 38s
Test suite with py38-windows-64	26m 58s
Test suite with py39-ubuntu	19m 52s
Test suite with py310-ubuntu	18m 21s
Test suite with py311-ubuntu, mypyc-compiled	17m 48s
Test suite with py312-ubuntu, mypyc-compiled	22m 36s
mypyc runtime tests with py39-macos	5m 39s
mypyc runtime tests with py38-debug-build-ubuntu	5m 22s
Type check our own code (py38-ubuntu)	1m 53s
Type check our own code (py38-windows-64)	2m 28s
Formatting and code style with Black + ruff	36s
	2h 29m 40s

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