Fast check unable to infer type from `return` expression

[marvinhagemeister]

These cases are trivial to detect based on the literal of the return expression. We currently fail all of them with a "missing retunr type" error whereas we be able to detect all of these.

export function strReturn() {
  return "foo";
}

export async function asyncStrReturn() {
  return "foo";
}

export function numReturn() {
  return 2;
}

export async function asyncNumReturn() {
  return 2;
}

[lucacasonato]

https://github.com/denoland/deno_graph/issues/459