DataFlow: Include PathGraph in GlobalFlowSig

This aims to remove the need for `::PathGraph` in common cases, so users just have to import the result of Global<...>.

Author: asgerf

shared/dataflow/codeql/dataflow/DataFlow.qll

@@ -615,7 +615,16 @@ module DataFlowMake<LocationSig Location, InputSig<Location> Lang> {
      * A `Node` augmented with a call context (except for sinks) and an access path.
      * Only those `PathNode`s that are reachable from a source, and which can reach a sink, are generated.
      */
-    class PathNode;
+    class PathNode {
+      /** Gets a textual representation of this element. */
+      string toString();
+
+      /** Gets the underlying `Node`. */
+      Node getNode();
+
+      /** Gets the location of this node. */
+      Location getLocation();
+    }
 
     /**
      * Holds if data can flow from `source` to `sink`.
@@ -639,6 +648,19 @@ module DataFlowMake<LocationSig Location, InputSig<Location> Lang> {
      * Holds if data can flow from some source to `sink`.
      */
     predicate flowToExpr(DataFlowExpr sink);
+
+    /** Holds if `(a,b)` is an edge in the graph of data flow path explanations. */
+    predicate edges(PathNode a, PathNode b, string key, string val);
+
+    /** Holds if `n` is a node in the graph of data flow path explanations. */
+    predicate nodes(PathNode n, string key, string val);
+
+    /**
+     * Holds if `(arg, par, ret, out)` forms a subpath-tuple, that is, flow through
+     * a subpath between `par` and `ret` with the connecting edges `arg -> par` and
+     * `ret -> out` is summarized as the edge `arg -> out`.
+     */
+    predicate subpaths(PathNode arg, PathNode par, PathNode ret, PathNode out);
   }
 
   /**

shared/dataflow/codeql/dataflow/internal/DataFlowImpl.qll

@@ -168,6 +168,8 @@ module MakeImpl<LocationSig Location, InputSig<Location> Lang> {
   module Impl<FullStateConfigSig Config> {
     private class FlowState = Config::FlowState;
 
+    import PathGraph
+
     private module SourceSinkFiltering {
       private import codeql.util.AlertFiltering