Either understand const arrow assertions or give better errors on them

[DanielRosenwasser]

As mentioned by @threehams in #35838 (comment), people are pretty confused about the rules for assertion functions, and I don't blame them. For example

const assert = (blah: unknown): asserts blah => { throw "hai"; }

let x: string | undefined;;
assert(x);

In our nightly releases, we give an elaboration on the declaration of assert like

'assert' needs an explicit type annotation.

For all intents and purposes, it looks like assert does have an annotation! The problem is that assert itself needs a : (x: unknown) => asserts x, because it's not automatically inferred from the arrow function.

We have two options:

• Remove this restriction when the declaration of an assertion function is trivially detectable.

• Make assertion function errors more clear for arrow functions:

  'assert' needs an explicit type annotation. While the function it has been assigned to is fully annotated, the variable declaration for 'assert' does not.
  

[flybayer]

This is still very confusing. Would be nice to get it fixed! :)

[ssalbdivad]

I constantly get tripped up by this! Strong +1 for:

Remove this restriction when the declaration of an assertion function is trivially detectable.

It doesn't seem like there should be a lot of edge cases to handle here if it's required that the return type is explicitly annotated.

Also worth noting this affects never returning functions as well, see #56362