Allow bitwise test on `undefined`

TypeScript Version: 2.8.0-dev.20180228

Search Terms: undefined strictNullChecks bitwise and or

(ref: #22088 (comment))

Code

const enum Flags { A = 2 ** 1, B = 2 ** 2 }
function f(flags?: Flags) {
    if (flags & Flags.B) {
    }
}

Expected behavior:

No error. & and | work well on undefined values: undefined | 2 is 2, undefined & 2 is 0. Contrast with undefined + 2 which is NaN.

Actual behavior:

src/a.ts(3,9): error TS2532: Object is possibly 'undefined'.

[RyanCavanaugh]

It's possible 0 and undefined are intended to have the same semantics in this function, but we really don't know. Arguably the signature should be flags: Flags = 0 if that's the case