New semantic analyzer: some assignments aren't recognized as types if incomplete

In the following test case the forward reference to In in the class body doesn't work:

[case testNewAnalyzerNamedTupleCallNested]
from typing import NamedTuple
o: C.Out
i: C.In
reveal_type(o)  # E: Revealed type is 'Tuple[Tuple[builtins.str, __main__.C.Other, fallback=__main__.C.In], __main__.C.Other, fallback=__main__.C.Out]'
reveal_type(o.x)  # E: Revealed type is 'Tuple[builtins.str, __main__.C.Other, fallback=__main__.C.In]'
reveal_type(o.y)  # E: Revealed type is '__main__.C.Other'
reveal_type(o.x.t)  # E: Revealed type is '__main__.C.Other'
reveal_type(i.t)  # E: Revealed type is '__main__.C.Other'

class C:
    Out = NamedTuple('Out', [('x', In), ('y', Other)])   # 'In' undefined  <<--- should be ok?
    In = NamedTuple('In', [('s', str), ('t', Other)])
    class Other: pass

The root cause is that if the rvalue of an assignment is incomplete, we add a PlaceholderNode that doesn't have becomes_typeinfo set. This may also affect other things such as assignment-based TypedDicts, type aliases, type variables and NewTypes (haven't checked).

Not sure what would be the best way to fix this. One idea would be to treat every placeholder as a possible type, at least if it's generated from an incomplete assignment. A second idea would be to recognize possible types by looking at an incomplete rvalue, but this doesn't seem always possible.

This is follow-up to #6983.

Commencez par exécuter le cas de test fourni pour testNewAnalyzerNamedTupleCallNested et examinez comment l’affectation incomplète crée un PlaceholderNode. Vérifiez si le même comportement affecte les TypedDicts fondés sur des affectations, les alias de type, les variables de type ou les NewTypes, comme indiqué dans l’issue. Le travail est terminé lorsque la référence anticipée est résolue et que les attentes reveal_type listées sont satisfaites sans régression dans les cas associés.