[ty] Fix narrowing enum literal unions by member identity (#25520)
## Summary
Prior to this change, we failed to narrow a union containing an
explicitly annotated enum literal (e.g., `x: list[int] |
Literal[Answer.NO]`) when comparing it against the corresponding enum
member:
```python
from enum import Enum
from typing import Literal
class Answer(Enum):
NO = 0
YES = 1
def f(x: list[int] | Literal[Answer.NO]):
if x is Answer.NO:
return
# Before: list[int] | Literal[Answer.NO]
# After: list[int]
reveal_type(x)
```
We were comparing the outer literal types, so the intersection builder
failed to recognize that both types represented the
same enum member.
Now, we compare the underlying enum-literal identities instead,
restoring narrowing for `is`, `is not`, `==`, and `!=`.
Closes https://github.com/astral-sh/ty/issues/3606.
