Exclude irrelevant members in narrow_declared_type from union overlapping with enum

[sterliakov]

Fixes #18895.

The original implementation of that block was introduced as a performance optimization in #12032. It's in fact incorrect: it produces overly optimistic meets, assuming that any match among union items makes them all relevant. As discussed in #18895, this actually results in unexpected meet behaviour, as demonstrated by

from enum import Enum
from typing_extensions import TypeIs, Literal

class Model(str, Enum):
    A = 'a'
    B = 'a'

def is_model_a(model: str) -> TypeIs[Literal[Model.A, "foo"]]:
    return True
def handle(model: Model) -> None:
    if is_model_a(model):
        reveal_type(model)  # N: Revealed type is "Union[Literal[__main__.Model.A], Literal['foo']]"


def is_int_or_list(model: object) -> TypeIs[int | list[int]]:
    return True
def compare(x: int | str) -> None:
    if is_int_or_list(x):
        reveal_type(x)  # N: Revealed type is "builtins.int"

This patch restores filtering of union members, but keeps it running before the expensive is_overlapping_types check involving expansion.

[sterliakov]

@sobolevn I remember you were doing some enums recently and you reviewed #12032, may I summon you to have a look? This PR is a crash fix.

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅

[sobolevn]

Thanks! Let's keep this open for a day, then merge :)