Warn about non-optional conditions that don't provide __nonzero__?

Maybe mypy should complain about Iterable[...] as an if condition, for example, since it generally doesn't define __nonzero__, so the check is likely unsafe? More generally, we could do this for all classes that don't define __nonzero__, but this could generate too many errors. One option would be to make this an optional check enabled by --strict, at least initially.

Examples:

def f(x: Iterable[int]) -> None:
    if not x:  # Might never be taken, or at least this could be inconsistently taken
        print('x')

class A:
    pass  # No __nonzero__

def g(a: A) -> None:
    if not a:   # Similar to above? What if there are subclasses of 'A'?
        print('x')

The issue names no files, tests, or entry points. Start with the Iterable[int] and class A examples, then review how mypy currently handles conditions and strict-mode checks. Done would require a settled scope for the diagnostic, including subclass behavior and whether it is optional or enabled by --strict.