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

首先复现 issue 中的两个 TypeScript 示例，并检查编译器对涉及字符串联合类型以及 null 或 undefined 的严格相等比较的类型检查行为。完成的标准是两个不可能的比较都会产生类型错误，而有效的比较仍被接受；payload 未指明源文件或测试。