Annotate immediately-invoked functions for inlining flow control analysis

Per #9998 and its many offshoots, we have problems analyzing code like this

let x: string | number = "OK";
mystery(() => {
  x = 10;
});
if (x === 10) { // Error, we believe this to be impossible  
}

The problem is that we don't know if mystery invokes its argument "now" or "later" (or possibly even never!).

The converse problem appears during reads:

let x: string | number = ...;
if (typeof x === 'string') {
  // Error, toLowerCase doesn't exist on string | number
  let arr = [1, 2, 3].map(n => x.toLowerCase());
}

Proposal is to allow some form of indicating that the function is invoked immediately, e.g.

declare function mystery(arg: immediate () => void): void;

This would "inline" the function body for the purposes of flow control analysis

Begin with the two TypeScript examples in the issue and trace how flow-control analysis handles writes and narrowed reads inside callbacks. Identify the type-checker entry points involved and determine the intended annotation semantics; done means immediately invoked callbacks are analyzed in the surrounding flow while later or never-invoked callbacks are not.