[used before def] improve handling of global definitions in local scopes

[ilinum]

While working on #14483, we discovered that variable inheritance didn't work quite right. In particular, functions would inherit variables from outer scope. On the surface, this is what you want but actually, they only inherit the scope if there isn't a colliding definition within that scope.

Here's an example:

class c: pass

def f0() -> None:
    s = c()  # UnboundLocalError is raised when this code is executed.
    class c: pass

def f1() -> None:
    s = c()  # No error.

This PR also fixes issues with builtins (exactly the same example as above but instead of c we have a builtin).

Fixes #14213 (as much as is reasonable to do)

[ilinum]

This didn't matter until we needed to handle generators differently. Generators actually do inherit the scope!

[ilinum]

when I wrote these import tests, I misunderstood how they would actually behave at runtime.

[JukkaL]

I wonder if this might degrade performance, since builtins have over 200 definitions, and it looks like we are iterating over all of them for every module. Can you measure the time spent in this loop when performing a self check, for example?

[ilinum]

I have found a way to deal with this that doesn't involve calling record_definition every time! So performance should be unchanged.

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉

[JukkaL]

LGTM. Thanks for making this faster!