Added error code for overlapping function signatures #13160

Job	Run time
Test mypyc suite with 32-bit Python	1m 6s
Test suite with Python nightly	1m 19s
Test suite with py38-ubuntu, mypyc-compiled	1m 19s
Test suite with py38-windows-64	1m 4s
Test suite with py39-ubuntu	1m 18s
Test suite with py310-ubuntu	1m 18s
Test suite with py311-ubuntu, mypyc-compiled	1m 17s
Test suite with py312-ubuntu, mypyc-compiled	1m 17s
mypyc runtime tests with py39-macos	1m 16s
mypyc runtime tests with py38-debug-build-ubuntu	1m 23s
Type check our own code (py38-ubuntu)	1m 17s
Type check our own code (py38-windows-64)	1m 4s
Formatting and code style with Black + ruff	36s
	15m 34s

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