[used-before-assignment] Functions defined in type-checking guard not flagged when used later

[jacobtylerwalls]

In #9990, we discovered that extending the fix for false negatives for used-before-assignment for objects guarded behind type-checking blocks to include functions would cause a handful of false positives in the primer. So we backed out that change and opened this issue to investigate.

@zenlyj graciously offered to find time to look into it.

[zenlyj]

will also look at this unused-import false negative that @nickdrozd found.

from typing import TYPE_CHECKING

if TYPE_CHECKING:
    from math import pi  # unused-import false negative