Go: Switch from def-use flow to use-use flow

[owen-mc]

This is a resurrection of github/codeql-go#460. Rebasing seems to have gone okay. There are more test changes. I am currently going through them to see if they are expected or if they indicate that something need to change.

[owen-mc]

There are two lost results for go/ql/src/InconsistentCode/UnhandledCloseWritableHandle.ql on go/ql/test/query-tests/InconsistentCode/UnhandledCloseWritableHandle/tests.go. They have the exact same cause: in the following function

func closeFileDeferredIndirect(f *os.File) {
	var cont = func() {
		f.Close() // NOT OK, if `f` is writable
	}

	defer cont()
}

we don't have flow from capture variable f in the function literal to the use of f in f.Close(). This flow used to be caught by this disjunct in basicLocalFlowStep:

  // SSA -> Instruction
  exists(SsaDefinition pred, IR::Instruction succ |
    succ = pred.getVariable().getAUse() and
    nodeFrom = ssaNode(pred) and
    nodeTo = instructionNode(succ)
  )

but now that has changed to:

  // SSA defn -> first SSA use
  exists(SsaExplicitDefinition pred, IR::Instruction succ | succ = pred.getAFirstUse() |
    nodeFrom = ssaNode(pred) and
    nodeTo = instructionNode(succ)
  )
  or
  // SSA use -> successive SSA use
  // Note this case includes Phi node traversal
  exists(IR::Instruction pred, IR::Instruction succ | succ = getAnAdjacentUse(pred) |
    nodeFrom = instructionNode(pred) and
    nodeTo = instructionNode(succ)
  )

capture variable f is an SsaImplicitDefinition rather than an SsaExplicitDefinition, so the first of these disjuncts doesn't catch it. I am not sure which of these disjuncts should be amended to capture this flow.

[smowton]

Presumably this is because of switching from SsaDefinition to SsaExplicitDefintion, I suppose with intent to exclude phi nodes, but also excluding SsaVariableCapture by mistake.