Bring typeof switch behaviour inline with if

[jack-williams]

This PR has now morphed into something that bundles a couples of changes to essentially bring switch narrowing inline with if, which was recently updated in two keys ways:

1. unknown narrowing for "function" and "object"
2. narrowing aswell as filtering for union types.

I don't know if this PR should be split in separate requests tackling different issues. This can probably be resolved after the release of 3.1 when 3.2 is more relevant.

Fixes #27180
Fixes #27335
Adds tests for #27181

[jack-williams]

I'm tempted to re-open this PR with a fresh one to tidy up all the history into a single commit. Just give me a thumbs up / response and I'll sort it out, link to the new one, and close this.

[justingrant]

@jack-williams @RyanCavanaugh - will this PR remove the compile error from the code below? Or will it only affect switch (typeof) and not other kinds of switch expressions? I assume the latter, but wanted to check before opening a dupe issue.

// compiled with strictNullChecks
function repro (x: string[] | null) { 

  switch (x && x.length) {
    case 1: 
      console.log (x[0]); // compiler error: Object is possibly null
      break;
    default: 
      console.log ('invalid x'); 
      break;
  }

  if (x && x.length===1) {
    console.log (x[0]); // no compiler error
  } else {
    console.log ('invalid x'); 
  }

}

[jack-williams]

@justingrant

I assume the latter, but wanted to check before opening a dupe issue.

You're right, this PR will only affect switch (typeof x) I'm afraid.

[justingrant]

OK thanks @jack-williams. After some more searching, it looks like the problem I ran into was #24091.

[jack-williams]

@RyanCavanaugh

Closing in favour of #27680