Menus: Ensure theme location setting data is saved with a large menu.

[36510] introduced a fix to allow users to save large menus on the Edit Menu screen (70+ menu items). The form data is stored as JSON which is used to inject the `$_POST` variable in the form submission handler. This injection was not setting form items with named array elements properly (e.g. `menu-locations[primary]`), which did not save theme location data in a large form.

Props keraweb.
See #14134.

Built from https://develop.svn.wordpress.org/trunk@36852


git-svn-id: https://core.svn.wordpress.org/trunk@36819 1a063a9b-81f0-0310-95a4-ce76da25c4cd