cmf: add idx() as a builtin
cmf: add idx() as a builtin

Summary:
Adding a builtin as documented at
https://our.intern.facebook.com/intern/wiki/index.php/Pfff/CheckModuleFast#FAQ

Test Plan: ./checkModule -test still works (no parse error in the builtins/).

Reviewers: rmcelroy, ahupp, julienv

Reviewed By: julienv

CC: julienv

Differential Revision: 366321

Task ID: 816259

mutantzombie/pfff,rpau/pfff,rpau/pfff,Hyunha/pfff,rpau/pfff,juergenhoetzel/pfff,mutantzombie/pfff,Hyunha/pfff,rpau/pfff,mutantzombie/pfff,juergenhoetzel/pfff,juergenhoetzel/pfff,juergenhoetzel/pfff,egonSchiele/pfff,Hyunha/pfff,egonSchiele/pfff,Hyunha/pfff,Hyunha/pfff,mutantzombie/pfff,juergenhoetzel/pfff,egonSchiele/pfff,juergenhoetzel/pfff,juergenhoetzel/pfff,Hyunha/pfff,mutantzombie/pfff,Hyunha/pfff,egonSchiele/pfff,egonSchiele/pfff,Hyunha/pfff,egonSchiele/pfff,mutantzombie/pfff,egonSchiele/pfff,egonSchiele/pfff,rpau/pfff,mutantzombie/pfff,rpau/pfff,rpau/pfff,juergenhoetzel/pfff,rpau/pfff,mutantzombie/pfff,Hyunha/pfff,juergenhoetzel/pfff,Hyunha/pfff,rpau/pfff,juergenhoetzel/pfff,mutantzombie/pfff,egonSchiele/pfff,egonSchiele/pfff,mutantzombie/pfff,mutantzombie/pfff,rpau/pfff