Introduce is_post_type_hierarchical()

[ptahdunbar]

Patch adds a new is_post_type_hierarchical() function to wp-includes/post.php.

It accepts one parameter which can be the post type name, id, or a post object and it will return a boolean result.

(In [14053]) Introduce is_post_type_hierarchical(). Props ptahdunbar. See #12950

Leaving this open for implementation throughout WordPress.

I changed the patch slightly, adding a few sanity checks to prevent PHP Warnings (Accessing non-existant properties, attempting to use a object as a array indicy, etc)

(In [14155]) Use is_post_type_hierarchical and is_taxonomy_hierarchical when we're fetching a post/tax object only to check the hierarchical flag. fixes #12950.

These were found in a search for '->hierarchical'. I didn't switch over any hierarchical checks when we used the post type object to check other properties.