Refactor parial types to avoid code duplication

Currently there is some duplication around partial types:

• Logic in try_infer_partial_generic_type_from_assignment() essentially duplicates the logic inlined in check_assignment() for None partial types. These two probably can be refactored to the same method if we also update handle_partial_var_type() to not special-case partial None types, and instead consistently return a partial type in lvalue context.
• Logic in try_infer_partial_type_from_indexed_assignment() duplicates that in try_infer_partial_type() (there is already a TODO item). This one can be refactored by either pushing the latter a bit down the call stack (closer to check_call()), or by generating a synthetic CallExpr with __setitem__ and passing it to try_infer_partial_type().

The second item may be something to watch out when implementing support for these:

a = defaultdict(list)
a[0].append('yes')

b = {}
b.setdefault(0, []).append('yes')

c = defaultdict(set)
c[0].add('yes')

d = {}
d.setdefault(0, set()).add('yes')

@JukkaL this is probably not something important, but maybe it makes sense to fix this while we are at it?

まず try_infer_partial_generic_type_from_assignment()、check_assignment()、handle_partial_var_type()、try_infer_partial_type_from_indexed_assignment()、try_infer_partial_type() の場所を確認します。それらの部分型の処理を比較し、続いて defaultdict と setdefault の例をリグレッションケースとして評価します。これらの推論シナリオを壊すことなく重複したロジックが統合されていれば完了です。