Propagate reads on global variables

[charliermarsh]

Summary

This PR ensures that if a variable is bound via global, and then the global is read, the originating variable is also marked as read. It's not perfect, in that it won't detect rebindings, like:

from app import redis_connection

def func():
    global redis_connection

    redis_connection = 1
    redis_connection()

So, above, redis_connection is still marked as unused.

But it does avoid flagging redis_connection as unused in:

from app import redis_connection

def func():
    global redis_connection

    redis_connection()

Closes #11518.

[github-actions]

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