C++: Add a sample class in PrintAST.ql

I've found myself typing out this class whenever I want to print the AST
of one function. I hope it will be useful to others too.

cpp/ql/src/semmle/code/cpp/PrintAST.ql

@@ -7,3 +7,12 @@
 
 import cpp
 import PrintAST
+
+/**
+ * Temporarily tweak this class or make a copy to control which functions are
+ * printed.
+ */
+class Cfg extends PrintASTConfiguration {
+  /** Holds if the AST for `func` should be printed. */
+  override predicate shouldPrintFunction(Function func) { any() }
+}