Add the printAst.ql contextual query for C++

This query will be used by the VS Code extension for viewing
ASTs of C/C++ files.

Author: aeisenberg

cpp/ql/src/printAst.ql

@@ -0,0 +1,27 @@
+/**
+ * @name Print AST
+ * @description Outputs a representation of a file's Abstract Syntax Tree. This
+ *              query is used by the VS Code extension.
+ * @id cpp/print-ast
+ * @kind graph
+ * @tags ide-contextual-queries/print-ast
+ */
+
+import cpp
+import semmle.code.cpp.PrintAST
+import definitions
+
+/**
+ * The source file to generate an AST from.
+ */
+external string selectedSourceFile();
+
+class Cfg extends PrintASTConfiguration {
+  /**
+   * Holds if the AST for `func` should be printed.
+   * Print All functions from the selected file.
+   */
+  override predicate shouldPrintFunction(Function func) {
+    func.getFile() = getEncodedFile(selectedSourceFile())
+  }
+}