Narrow the parent type of a singleton type to never in the false branch

If the type of the member checked is not falsy (and so is `never` in the false branch). <!-- Please try to reproduce the issue with `typescript@next`. It may have already been fixed. --> **TypeScript Version:** 2.7.0-dev.201xxxxx **Code** ```ts const x: {a: 1, b: 2} = { a: 1, b: 2 }; if (x.a) { } else { x.a; } ``` **Expected behavior:** type of `x` in the `else` branch is `never`. **Actual behavior:** type of `x` is `{a: 1. b: 2}`, and `x.a` is (counterintuitively) `never`. This would bring the behavior of truthiness guards in line with our new behavior for `in` type guards.