C++: Map some indirect nodes to expressions in localExprFlowStep

[MathiasVP]

This PR supersede #12477 and (hopefully) contains the correct performance fix.

On certain projects containing heavily macro-expanded code like:

*x = source();
use(x[0]);
use(x[1]);
...
use(x[i]);
use(x[i+1]);
...
use(x[N]);

we hit a performance problem caused by each expression of the form x[i] being stepped to from *x = source() in certain cases in the expression-only local flow relation.

To see why this is the case, consider the transitive closure over localStepFromNonExpr in localStepsToExpr. We start at n2 for which n.asExpr() exists. For example, n2 in the above example could be x[i] in any of the use(x[i]) above. We then step to a dataflow predecessor of n2. In the above code fragment, thats the indirect node corresponding to x in x[i-1]. Since this doesn't have a result for Node::asExpr() we continue with the recursion until we reach *x = source() which does have a result for Node::asExpr().

If N is very large this blows up.

This PR fixes this problem by mapping those indirect nodes for x in x[i] to the x[i] expression. Ideally, this would be handled automatically by assigning the dataflow node for the indirection of x to the same dataflow node as x[i], but doing this in the past has let to some strange conflations. However, by just doing this in the expression-only local flow relation we're not seeing these strange conflations.

[MathiasVP]

Like in the previous version of this PR, this change is the result of me running the new autoformatter on this file which hasn't been updated to include the latest autoformatter changes.

[jketema]

LGTM if DCA is happy.