Swift: add summary about total extracted nodes

swift/ql/src/diagnostics/internal/AstNodes.ql

@@ -0,0 +1,11 @@
+/**
+ * @name Number of extracted AST nodes
+ * @description Count all AST nodes.
+ * @kind metric
+ * @id swift/diagnostics/ast-nodes
+ * @tags summary
+ */
+
+import swift
+
+select count(AstNode n)