Unreachability should work for constrained type vars

Feature

Unreachability should be checked for constrained type vars.

Pitch

It would be nice if this raised an error when run with --warn-unreachable:

from typing import TypeVar

U = TypeVar('U', int, str)

def f(u: U) -> U:
    if u is None:
        print("whoa!!")
    
    return u

To implement this feature, I think we need to store the spans of unreachability for each run, then find the intersection. I thought for a while that we could just treat constrained type variables as if they were bound to the union of constraints, but things like https://github.com/python/mypy/issues/9424 will happen for things that aren't final (unlike None).