Ensure defs from `TYPE_CHECKING` also exist in the else branch

[KotlinIsland]

if TYPE_CHECKING:
    a = 1  # no corresponding real definition
print(a)

Of course this doesn't facilitate all usages:

if TYPE_CHECKING:
    from _typedshed import SupportsAmongUs
a: SupportsAmongUs  # only used as type, so it's ok

But really, that usage should be handled better, like with a type import.