Literal promotion is not performed for generic calls

For example, on main:
```py
def lst[T](x: T) -> list[T]:
    return [x]

a = lst(1)
reveal_type(a)  # revealed: list[Literal[1]]
```

We currently perform literal promotion when inferring the list expression, i.e. before specialization. We may need a second literal promotion step during specialization for this to work, similar to what we do for generic constructors.