Un-blacklist PhanUndeclaredVariable

Un-blacklist PhanUndeclaredVariable

Undeclared variables are a very common error type that we want to catch
as often as possible. To avoid needing to refactor a variety of global
level code (mostly in old-style maintenance scripts) this ignores
undeclared variables in global scope. This is still a good improvement
over what was happening previously.

Change-Id: I50b41d571724244552074b9408abbdf6160aca59