Regression: false positive avoid redundant cast with literal union

This example started generating an error after #18588, which looks like a regression:

# mypy: warn-redundant-casts

from typing import Literal, cast

Foo = Literal["a", "b"]

class C:
    def __init__(self) -> None:
        self.x = cast(Foo, "a")  # error: Redundant cast to "Literal['a', 'b']"

I haven't looked into this in detail, but it seems possible that we infer a union type for "a" in a literal union context, which doesn't look right. If this is a correct hypothesis, #18588 wouldn't actually be the root cause, and it would just be exposing a pre-existing issue.

cc @asottile as the author of the PR

Start by reproducing the example in issue #19055 with mypy's warn-redundant-casts setting, then inspect the behavior introduced by #18588 and the literal-union inference path. Done means the cast in the example is no longer reported as redundant, with a regression test covering this case.