Generalize `never` type handling for control flow analysis based type guard

I guess there would be some duplicates, but I cannot find it.

TypeScript Version: 2.1.14

Code

function throwError(): never {
    throw new Error();
}

let foo: string | undefined;

if (!foo) {
    throwError();
}

foo; // Expected to be `string` instead of `string | undefined`.

[simonbuchan]

For other people googling this, this issue was addressed in #14490. Basically it seems the issue is TS currently has separate passes for flow control and type assignment, and the former needs to run first for the latter to work, and there didn’t seem to be any clean solutions that would also handle imported functions, etc… So the current workaround is just return neverReturningFunction();, which won’t alter the return type and lets flow control do its thing.

[masaeedu]

I currently use const guaranteedFoo = foo || throwError() to strip away undefined from stuff that I know should never be undefined.