Generator without key should allow key type `int`

<?php
/** @return Generator<void, string> */
function generator () { yield 'x'; yield 'y'; };
// Observed: PhanTypeMismatchGeneratorYieldKey Yield statement has a key with type void but generator2() is declared to yield keys of type int in \Generator<int,string>
/** @return Generator<int, string> */
function generator2 () { yield 'x'; yield 'y'; };

php > $gen = function () { yield 'x'; };
php > foreach ($gen() as $i => $value) { var_export([$i, $value]); }            
array (
  0 => 0,
  1 => 'x',
)
php > $gen = function () { yield 'x'; yield 'y'; };                             
php > foreach ($gen() as $i => $value) { var_export([$i, $value]); }
array (
  0 => 0,
  1 => 'x',
)array (
  0 => 1,
  1 => 'y',
)
php > var_export(iterator_to_array($gen()));
array (
  0 => 'x',
  1 => 'y',
)

Reproduce the two generator examples and search the codebase for PhanTypeMismatchGeneratorYieldKey and Generator key-type validation. Check how omitted yield keys are represented when a generator is declared with an int key type; done means the second example no longer reports a mismatch while the documented key and value types remain checked.