C++: Improve PrintAST performance.

This improves the performance of the printAst.ql query by excluding a lot of string concatenations that happen in files unrelated to the one the user is interested in printing.
This is supposed to help the performance of the AST Viewer on bigger databases.

Author: criemen

cpp/ql/src/semmle/code/cpp/Print.qll

@@ -1,4 +1,17 @@
 import cpp
+private import PrintAST
+
+/**
+ * Print function declarations only if there is a `PrintASTConfiguration`
+ * that requests that function, or no `PrintASTConfiguration` exists.
+ */
+private predicate shouldPrintDeclaration(Declaration decl) {
+  not decl instanceof Function
+  or
+  not exists(PrintASTConfiguration c)
+  or
+  exists(PrintASTConfiguration config | config.shouldPrintFunction(decl))
+}
 
 /**
  * Gets a string containing the scope in which this declaration is declared.
@@ -48,6 +61,8 @@ private string getTemplateArgumentString(Declaration d, int i) {
  * A `Declaration` extended to add methods for generating strings useful only for dumps and debugging.
  */
 abstract private class DumpDeclaration extends Declaration {
+  DumpDeclaration() { shouldPrintDeclaration(this) }
+
   /**
    * Gets a string that uniquely identifies this declaration, suitable for use when debugging queries. Only holds for
    * functions, user-defined types, global and namespace-scope variables, and member variables.