C++: Add flow through arrays

This works by adding data-flow edges to skip over array expressions when
reading from arrays. On the post-update side, there was already code to
skip over array expressions when storing to arrays. That happens in
`valueToUpdate` in `AddressFlow.qll`, which needed just a small tweak to
support assignments with non-field expressions at the top-level LHS,
like `*a = ...` or `a[0] = ...`.

The new code in `AddressFlow.qll` is copy-pasted from `EscapesTree.qll`,
and there is already a note in these files saying that they share a lot
of code and must be maintained in sync.

cpp/ql/test/library-tests/dataflow/fields/by_reference.cpp

@@ -109,11 +109,11 @@ void test_outer_with_ptr(Outer *pouter) {
 
   sink(outer.inner_nested.a); // $ast,ir
   sink(outer.inner_ptr->a); // $ast $f-:ir
-  sink(outer.a); // $f-:ast,ir
+  sink(outer.a); // $ast $f-:ir
 
   sink(pouter->inner_nested.a); // $ast,ir
   sink(pouter->inner_ptr->a); // $ast $f-:ir
-  sink(pouter->a); // $f-:ast,ir
+  sink(pouter->a); // $ast $f-:ir
 }
 
 void test_outer_with_ref(Outer *pouter) {