Fix crash on access to local classes

[ilevkivskyi]

Fixes #2559

The idea is the same as in PR #2553. The only difference is that mangled names are always stored in globals, otherwise there would be a problem for a method in a nested class that is itself inside a method.

[ilevkivskyi]

@gvanrossum If you are curious, this monster still crashes when imported from elsewhere in incremental mode:

from typing import NamedTuple

class C:
    def f(self) -> None:
        class D:
            class E:
                def g(self) -> None:
                    A = NamedTuple('A', [('x', int), ('y', int)])
                    self.a = A(1, 1)
        self.c = D.E().a

This is why I made this PR to always use globals. Such approach is more robust against such deeply nested classes. In addition, always using globals simplifies code without risk of conflicts (since line numbers are unique anyway).

[JukkaL]

Thanks for the fix!