New semantic analyzer: dummy alias definitions in typing stub (List etc.)

[JukkaL]

Support definitions for built-in type aliases List, Set in the stub for typing. Currently these don't work, which is a problem since typeshed includes these, whereas test fixtures don't.

[ilevkivskyi]

FWIW I just whitelisting some test cases to the new semantic analyzer, and it looks like many of them are blocked by this:

Actual:
  tmp/builtins.pyi:1: error: Module 'typing' has no attribute 'List' (diff)
  tmp/builtins.pyi:1: error: Module 'typing' has no attribute 'Set' (diff)

I will work on this next to unblock the situation.

[JukkaL]

Note that my current working branch makes changes to how the aliases are handled.