Added error code for overlapping function signatures #13162

Job	Run time
Test suite with Python nightly	13m 53s
Test suite with py38-ubuntu, mypyc-compiled	17m 45s
Test mypyc suite with 32-bit Python	6m 49s
Test suite with py38-windows-64	19m 59s
Test suite with py39-ubuntu	19m 31s
Test suite with py310-ubuntu	15m 33s
Test suite with py311-ubuntu, mypyc-compiled	15m 48s
Test suite with py312-ubuntu, mypyc-compiled	17m 35s
mypyc runtime tests with py39-macos	5m 7s
mypyc runtime tests with py38-debug-build-ubuntu	4m 22s
Type check our own code (py38-ubuntu)	1m 50s
Type check our own code (py38-windows-64)	2m 12s
Formatting and code style with Black + ruff	39s
	2h 21m 3s

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