raise a type error on attempt to strict-equal-compare a value of string | null to undefined

the following examples are almost certainly bugs:

declare var one: string | undefined;
// expected a type error: Operator '===' cannot be applied to types 'string | undefined' and 'null'
// actual no problem
if (one === null) { 
   // unreachable, because one can't ever be null, undefined was likely meant instead
}

declare var another: string | null;
// expected a type error: Operator '===' cannot be applied to types 'string | null' and 'undefined'
// actual no problem
if (another === undefined) {
   // unreachable, because one can't ever be undefined, null was likely meant instead
}

consider raising a type error for the above situations

Start by reproducing the two TypeScript examples in the issue and inspect the compiler's type-checking behavior for strict equality comparisons involving string unions and null or undefined. Done means both impossible comparisons produce type errors while valid comparisons remain accepted; the payload does not name a source file or test.