Infer unions for ternary expressions (#17427) #27

Job	Run time
Test mypyc suite with 32-bit Python	7m 0s
Test suite with py38-ubuntu, mypyc-compiled	17m 9s
Test suite with py38-windows-64	19m 56s
Test suite with py39-ubuntu	15m 59s
Test suite with py310-ubuntu	15m 27s
Test suite with py311-ubuntu, mypyc-compiled	15m 35s
Test suite with py312-ubuntu, mypyc-compiled	17m 34s
mypyc runtime tests with py39-macos	4m 54s
mypyc runtime tests with py38-debug-build-ubuntu	4m 22s
Type check our own code (py38-ubuntu)	1m 47s
Type check our own code (py38-windows-64)	2m 21s
Formatting and code style with Black + ruff	33s
	2h 2m 37s

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