C++: enable the QL-based CFG code

Author: jbj

change-notes/1.23/analysis-cpp.md

@@ -42,3 +42,7 @@ The following changes in version 1.23 affect C/C++ analysis in all applications.
   clarity (e.g. `isOutReturnPointer()` to `isReturnValueDeref()`). The existing member predicates
   have been deprecated, and will be removed in a future release. Code that uses the old member
   predicates should be updated to use the corresponding new member predicate.
+* The control-flow graph is now computed in QL, not in the extractor. This can
+  lead to regressions (or improvements) in how queries are optimized because
+  optimization in QL relies on static size estimates, and the control-flow edge
+  relations will now have different size estimates than before.

cpp/ql/src/semmle/code/cpp/controlflow/ControlFlowGraph.qll

@@ -1,6 +1,7 @@
 import cpp
 import BasicBlocks
 private import semmle.code.cpp.controlflow.internal.ConstantExprs
+private import semmle.code.cpp.controlflow.internal.CFG
 
 /**
  * A control-flow node is either a statement or an expression; in addition,
@@ -86,11 +87,11 @@ import ControlFlowGraphPublic
 class ControlFlowNodeBase extends ElementBase, @cfgnode { }
 
 predicate truecond_base(ControlFlowNodeBase n1, ControlFlowNodeBase n2) {
-  truecond(unresolveElement(n1), unresolveElement(n2))
+  qlCFGTrueSuccessor(n1, n2)
 }
 
 predicate falsecond_base(ControlFlowNodeBase n1, ControlFlowNodeBase n2) {
-  falsecond(unresolveElement(n1), unresolveElement(n2))
+  qlCFGFalseSuccessor(n1, n2)
 }
 
 /**
@@ -120,7 +121,7 @@ abstract class AdditionalControlFlowEdge extends ControlFlowNodeBase {
  * `AdditionalControlFlowEdge`. Use this relation instead of `successors`.
  */
 predicate successors_extended(ControlFlowNodeBase source, ControlFlowNodeBase target) {
-  successors(unresolveElement(source), unresolveElement(target))
+  qlCFGSuccessor(source, target)
   or
   source.(AdditionalControlFlowEdge).getAnEdgeTarget() = target
 }