Make ControlFlowChecker follow the dataflow analysis idiom more closely

This makes ControlFlowInfo an ADT handling its own join operation, and combines the list of return statements into that structure. This is less error-prone if a new field is added, and helps simplify the logic in visit(IfStatement).

The invariants regarding the cf field are also clarified: It is "transferred" (in dataflow analysis terminology) from the point before the node to the point after the node by the visit() function. It is also undefined (null) outside of a function definition.

There's a cost to this CL in terms of LOC and maybe even memory allocations, but I suspect that's outweighed by sticking to a known paradigm for flow analysis.

RELNOTES: None
PiperOrigin-RevId: 167005045

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