[red-knot] Add `FunctionType::to_overloaded` (#17585)
## Summary
This PR adds a new method `FunctionType::to_overloaded` which converts a
`FunctionType` into an `OverloadedFunction` which contains all the
`@overload`-ed `FunctionType` and the implementation `FunctionType` if
it exists.
There's a big caveat here (it's the way overloads work) which is that
this method can only "see" all the overloads that comes _before_ itself.
Consider the following example:
```py
from typing import overload
@overload
def foo() -> None: ...
@overload
def foo(x: int) -> int: ...
def foo(x: int | None) -> int | None:
return x
```
Here, when the `to_overloaded` method is invoked on the
1. first `foo` definition, it would only contain a single overload which
is itself and no implementation.
2. second `foo` definition, it would contain both overloads and still no
implementation
3. third `foo` definition, it would contain both overloads and the
implementation which is itself
### Usages
This method will be used in the logic for checking invalid overload
usages. It can also be used for #17541.
## Test Plan
Make sure that existing tests pass.
