sgrep: remove support for deprecated $V lvalue metavariable

Summary:
Now that lvalues and expressions are unified in ast_php.ml, there is
no more need for the special ugly $V lvalue metavariable in sgrep.
I've updated the wiki accordingly.
https://github.com/facebook/pfff/wiki/Sgrep#wiki-metavariables
https://github.com/facebook/pfff/wiki/Sgrep#wiki-how-to-match-method-calls

Test Plan:
~/pfff/sgrep -lang php -e '$V->foo()' foo.php
Fatal error: exception Failure("Lvalue metavariables are deprecated, just use expression metavariables as in X->foo()")

Reviewers: pieter, julienv

Reviewed By: pieter

CC: jiaol, hannesr, matthieu, bill, ahupp

Differential Revision: https://phabricator.fb.com/D842941
cmf: update list of builtins, handle the new Set class

Summary:
cd ~/local/fbcode/hphp/idl
$ ~/pfff/pfff_test -builtins_of_idl collections.idl.json  > ~/pfff/data/php_stdlib/builtins_collection.idl.php

Test Plan:
$ cmf flib/intern/web/haste/map
now works

no more "Undefined class Set"

Reviewers: pieter, jiaol, julienv, amenghra

Reviewed By: pieter

CC: akramer, alvins

Differential Revision: https://phabricator.fb.com/D843293