This adds type checking around the the new optional-chaining feature. With this, it becomes invalid to start an optional chain when the base value is known to be not-nullish.

```js
// Good
declare const foo: string | null;
declare const bar: null : (() => {});
foo?.length;
foo?.slice();
bar?.();

// Bad
declare const baz: string;
declare const qux: (() => {});
baz?.length;
baz?.slice();
baz.slice?.();
qux?.();
```