Support type narrowing literals using `typing.get_args()`

Feature

When using typing.get_args() to ensure that a given string is of a literal type, I would expect mypy to behave similarly than when isinstance() checks are used.

Pitch
Take the following code:

from typing import Literal, TypeAlias, get_args

ExpectedUserInput: TypeAlias = Literal[
    "these", "strings", "are", "expected", "user", "input"
]


def external_function(input: str) -> str:
    if input not in get_args(ExpectedUserInput):
        raise ValueError("Invalid input.")
    return _internal_function(input)


def _internal_function(input: ExpectedUserInput) -> str:
    return f"User input: {input}"

Mypy does not detect that the string must be of the correct type, since the code path is unreachable for an invalid value and outputs an error, requiring the use of casts even though the value is guaranteed to be of the correct type:

...: error: Argument 1 to "_internal_function" has incompatible type "str"; expected "Literal['these', 'strings', 'are', 'expected', 'user', 'input']"  [arg-type]

Start with the issue's get_args() and Literal example and investigate how mypy currently performs type narrowing for membership checks. Done means mypy recognizes input as the declared literal type after the invalid-input branch without requiring a cast, while still reporting invalid values appropriately.