QL: Add NodeNumbering library

ql/ql/src/codeql_ql/ast/internal/AstNodeNumbering.qll

@@ -0,0 +1,38 @@
+private import codeql_ql.ast.Ast
+
+pragma[inline]
+private predicate isNumberedNode(AstNode node) {
+  // these are not nested in the location of the parent node, so we can't use their location to order them
+  not node instanceof Annotation and
+  not node instanceof QLDoc
+}
+
+private int getNodeDepth(AstNode node) {
+  node instanceof TopLevel and
+  result = 0
+  or
+  isNumberedNode(node) and
+  result = 1 + getNodeDepth(node.getParent())
+}
+
+/**
+ * Gets the pre-order ID for the given `node`, that is, its visit position
+ * in a pre-order traversal of all nodes.
+ *
+ * The children of a node are ordered left-to-right as they appear in the source code.
+ *
+ * The ID is globally unique for this AST node, also across files.
+ *
+ * At the moment this predicate is only defined for node which are:
+ * - reachable via `getAChild` edges from a `TopLevel`, and
+ * - is not a comment or annotation
+ */
+cached
+int getPreOrderId(AstNode node) {
+  node =
+    rank[result](AstNode n, Location loc, int depth |
+      depth = getNodeDepth(n) and loc = n.getLocation()
+    |
+      n order by loc.getFile().getAbsolutePath(), loc.getStartLine(), loc.getStartColumn(), depth
+    )
+}