New semantic analyzer: Explain that global variable must be declared at module top level

The error message is poor for code like this where we don't initialize or declare a global variable at the module top level:

from typing import List


def foo() -> None:
    global bar
    bar = []  # type: List[str]  # Name 'bar' already defined (possibly by an import)
    bar  # Name 'bar' is not defined

It would be better to be explicit and say something like "Global variable must be declared or initialized at module top level".

Follow-up to #6975.

Start with the semantic analyzer handling global declarations and reproduce the Python snippet from the issue. Update the diagnostic so this case explains that the global variable must be declared or initialized at module top level, then verify that the reported message matches the requested wording.