Use get_queried_object_id() as a more reliable way to get wp_cache_post_id()

On our site the `wp_cache_post_id` function is throwing an error:

`PHP Fatal error: Cannot use object of type WP_Query as array in public_html/wp-content/plugins/wp-super-cache/wp-cache-phase2.php on line 1278`

This is the function. It turns out that the way the things are setup, while `is_page()` is true, `$posts[0]->ID` does not contain the expected variable.

```
function wp_cache_post_id() {
global $posts, $comment_post_ID, $post_ID;
// We try hard all options. More frequent first.
if ($post_ID > 0 ) return $post_ID;
if ($comment_post_ID > 0 ) return $comment_post_ID;
if (is_single() || is_page()) return $posts[0]->ID;
if (isset( $_GET[ 'p' ] ) && $_GET['p'] > 0) return $_GET['p'];
if (isset( $_POST[ 'p' ] ) && $_POST['p'] > 0) return $_POST['p'];
return 0;
}
```

If I change the function by first checking `if(get_queried_object_id()) return get_queried_object_id();`, our page loads fine without throwing the fatal error:

```
function wp_cache_post_id() {
global $posts, $comment_post_ID, $post_ID;
// We try hard all options. More frequent first.
if(get_queried_object_id()) return get_queried_object_id();
if ($post_ID > 0 ) return $post_ID;
if ($comment_post_ID > 0 ) return $comment_post_ID;
if (is_single() || is_page()) return $posts[0]->ID;
if (isset( $_GET[ 'p' ] ) && $_GET['p'] > 0) return $_GET['p'];
if (isset( $_POST[ 'p' ] ) && $_POST['p'] > 0) return $_POST['p'];
return 0;
}
```

Update: it turns out that the theme we were using was incorrectly overwriting the `$posts` variable for a separate query. I fixed that, however I think updating `wp_cache_post_id()` to use `get_queried_object_id()` as a first resort would still be a good idea.

Read wp-cache-phase2.php around line 1278 and inspect wp_cache_post_id(), including how its existing fallbacks use $posts. Verify the WordPress query context that caused the reported WP_Query-as-array error, then confirm the function reliably returns the queried post ID without breaking its fallback behavior.

Written by the indexing model from the issue text.