Hide 0Closure variable from getDefinedVariables

Summary: Closures have an internal variable "0Closure" that points to the last
defined function. Previously, calls to get_defined_vars would return
this (and php5 didn't). This modifies VarEnv::getDefinedVariables so
that the "0Closure" variable is not included.

Reviewed By: @jano

Differential Revision: D1401578