Make `narrow_declared_type` correctly handle Literal types (#6961)
This diff fixes the `narrowed_declared_type` function to correctly
handle cases where the declared type is actually a `Literal[...]`.
Specifically, doing something like the below will return `int` instead
of `Literal[42]`:
narrowed_declared_type(Literal[42], int)
This is inconsistent with the behavior of doing something like the
below, which will return `ChildType`:
narrowed_declared_type(ChildType, ParentType)
This diff modifies `narrowed_declared_type` to instead return `Literal[42]`
in the first example.
