C++/C#/Java: Split localFlowStep predicate in two

There's now a `localFlowStep` predicate for use directly in queries and
other libraries and a `simpleLocalFlowStep` for use only by the global
data flow library. The former predicate is intended to include field
flow, but the latter may not.

This will let Java and C# (and possibly C++ IR) avoid getting two kinds
of field flow at the same time, both from SSA and from the global data
flow library. It should let C++ AST add some form of field flow to
`localFlowStep` without making it an input to the global data flow
library.

java/ql/src/semmle/code/java/dataflow/internal/DataFlowImpl.qll

@@ -162,7 +162,7 @@ private predicate isAdditionalFlowStep(
  * Holds if data can flow in one local step from `node1` to `node2`.
  */
 private predicate localFlowStep(Node node1, Node node2, Configuration config) {
-  localFlowStep(node1, node2) and
+  simpleLocalFlowStep(node1, node2) and
   not outBarrier(node1, config) and
   not inBarrier(node2, config) and
   not fullBarrier(node1, config) and

java/ql/src/semmle/code/java/dataflow/internal/DataFlowImpl2.qll

@@ -162,7 +162,7 @@ private predicate isAdditionalFlowStep(
  * Holds if data can flow in one local step from `node1` to `node2`.
  */
 private predicate localFlowStep(Node node1, Node node2, Configuration config) {
-  localFlowStep(node1, node2) and
+  simpleLocalFlowStep(node1, node2) and
   not outBarrier(node1, config) and
   not inBarrier(node2, config) and
   not fullBarrier(node1, config) and

java/ql/src/semmle/code/java/dataflow/internal/DataFlowImpl3.qll

@@ -162,7 +162,7 @@ private predicate isAdditionalFlowStep(
  * Holds if data can flow in one local step from `node1` to `node2`.
  */
 private predicate localFlowStep(Node node1, Node node2, Configuration config) {
-  localFlowStep(node1, node2) and
+  simpleLocalFlowStep(node1, node2) and
   not outBarrier(node1, config) and
   not inBarrier(node2, config) and
   not fullBarrier(node1, config) and

java/ql/src/semmle/code/java/dataflow/internal/DataFlowImpl4.qll

@@ -162,7 +162,7 @@ private predicate isAdditionalFlowStep(
  * Holds if data can flow in one local step from `node1` to `node2`.
  */
 private predicate localFlowStep(Node node1, Node node2, Configuration config) {
-  localFlowStep(node1, node2) and
+  simpleLocalFlowStep(node1, node2) and
   not outBarrier(node1, config) and
   not inBarrier(node2, config) and
   not fullBarrier(node1, config) and

java/ql/src/semmle/code/java/dataflow/internal/DataFlowImpl5.qll

@@ -162,7 +162,7 @@ private predicate isAdditionalFlowStep(
  * Holds if data can flow in one local step from `node1` to `node2`.
  */
 private predicate localFlowStep(Node node1, Node node2, Configuration config) {
-  localFlowStep(node1, node2) and
+  simpleLocalFlowStep(node1, node2) and
   not outBarrier(node1, config) and
   not inBarrier(node2, config) and
   not fullBarrier(node1, config) and

java/ql/src/semmle/code/java/dataflow/internal/DataFlowUtil.qll

@@ -352,6 +352,17 @@ predicate hasNonlocalValue(FieldRead fr) {
  */
 cached
 predicate localFlowStep(Node node1, Node node2) {
+  simpleLocalFlowStep(node1, node2)
+}
+
+/**
+ * INTERNAL: do not use.
+ *
+ * This is the local flow predicate that's used as a building block in global
+ * data flow. It may have less flow than the `localFlowStep` predicate.
+ */
+cached
+predicate simpleLocalFlowStep(Node node1, Node node2) {
   // Variable flow steps through adjacent def-use and use-use pairs.
   exists(SsaExplicitUpdate upd |
     upd.getDefiningExpr().(VariableAssign).getSource() = node1.asExpr() or