stubgen: fix FunctionContext.fullname for nested classes (#17963) #13818

Job	Run time
Test mypyc suite with 32-bit Python	7m 12s
Test suite with py38-ubuntu, mypyc-compiled	18m 9s
Test suite with py38-windows-64	20m 46s
Test suite with py39-ubuntu	17m 38s
Test suite with py310-ubuntu	16m 28s
Test suite with py311-ubuntu, mypyc-compiled	15m 45s
Test suite with py312-ubuntu, mypyc-compiled	17m 53s
Test suite with py313-ubuntu, mypyc-compiled	18m 44s
mypyc runtime tests with py39-macos	4m 30s
Type check our own code (py38-ubuntu)	1m 54s
Type check our own code (py38-windows-64)	2m 27s
Formatting and code style with Black + ruff	55s
	2h 22m 21s

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