Move getUnderlyingNode into Protobuf.qll

This is its only user for now.

Author: smowton

ql/src/semmle/go/dataflow/internal/DataFlowUtil.qll

@@ -600,16 +600,6 @@ class ComponentReadNode extends ReadNode {
   Node getBase() { result = instructionNode(insn.getBase()) }
 }
 
-/**
- * Gets the data-flow node representing the bottom of a stack of zero or more `ComponentReadNode`s.
- *
- * For example, in the expression a.b[c].d[e], this would return the dataflow node for the read from `a`.
- */
-Node getUnderlyingNode(ReadNode read) {
-  (result = read or result = read.(ComponentReadNode).getBase+()) and
-  not result instanceof ComponentReadNode
-}
-
 /**
  * A data-flow node that reads an element of an array, map, slice or string.
  */

ql/src/semmle/go/frameworks/Protobuf.qll

@@ -145,16 +145,26 @@ module Protobuf {
     exists(Type base | base.getPointerType() instanceof MessageType | result = base.getField(_))
   }
 
+  /**
+   * Gets the data-flow node representing the bottom of a stack of zero or more `ComponentReadNode`s.
+   *
+   * For example, in the expression a.b[c].d[e], this would return the dataflow node for the read from `a`.
+   */
+  DataFlow::Node getUnderlyingNode(DataFlow::ReadNode read) {
+    (result = read or result = read.(DataFlow::ComponentReadNode).getBase+()) and
+    not result instanceof DataFlow::ComponentReadNode
+  }
+
   /**
    * Additional taint step tainting a Message when taint is written to any of its fields and/or elements.
    */
   private class WriteMessageFieldStep extends TaintTracking::AdditionalTaintStep {
     override predicate step(DataFlow::Node pred, DataFlow::Node succ) {
-      exists(DataFlow::ReadNode base | succ = DataFlow::getUnderlyingNode(base) |
+      exists(DataFlow::ReadNode base | succ = getUnderlyingNode(base) |
         any(DataFlow::Write w).writesField(base, getAMessageField(), pred)
       )
       or
-      exists(DataFlow::ReadNode base | succ = DataFlow::getUnderlyingNode(base) |
+      exists(DataFlow::ReadNode base | succ = getUnderlyingNode(base) |
         any(DataFlow::Write w).writesElement(base, _, pred) and
         [succ.getType(), succ.getType().getPointerType()] instanceof MessageType
       )