C++: IR data flow from WriteSideEffectInstructions to partial ChiInstruction operands

[MathiasVP]

Previously we did not catch data flows such as:

int a = source();
int b = 0;
memcpy(&b, &a, sizeof(int));
sink(b); // tainted

because there's a ChiInstruction between the WriteSideEffect instruction and the load in sink(b). (Note that this was a still caught by taint flow).

This PR recovers these flows by inserting flow from write side effect instructions to the partial operand of Chi instructions, similar to how #3219 handles taint flows.

[MathiasVP]

I've started a CPP-differences run to make sure we don't add too much flow: https://jenkins.internal.semmle.com/job/Changes/job/CPP-Differences/1026/

As @jbj pointed out, we have flow to partial chi operands already in DefaultTaintTracking. So there's no new query results as there's no query that uses IR dataflow without using taintflow.

[MathiasVP]

Closed as this PR was integrated into #3118.