Changed ExpectReturnOnAllPaths() to treat continuation, run and await as missing returns

[pk19604014]

Fixes #1257

[jonmeow]

I'm not sure this is correct: can you please add a test case of

fn Main() -> i32 {
  var x: i32 = 0;
  __continuation k {
    x = x + 1;
    return x;
  }
}

And make sure this fails due to the missing return?

[pk19604014]

I'm not sure this is correct: can you please add a test case of

fn Main() -> i32 {
  var x: i32 = 0;
  __continuation k {
    x = x + 1;
    return x;
  }
}

And make sure this fails due to the missing return?

Added fail_return_in_continuation.carbon. It fails both with and without the change with a 'return is not within a function body' error.