Comments: Update comment cache in the upgrade routine for changing the...

Comments: Update comment cache in the upgrade routine for changing the `comment_type` DB field value in comments table.

This ensures that comment object cache is cleared after changing the comment type to `comment` instead of an empty string.

Add a unit test for `_wp_batch_update_comment_type()`.

Follow-up to [47597], [47626], [48225], [48227].

Props imath, westonruter.
Reviewed by desrosj, SergeyBiryukov.
Merges [48748] and [48751] to the 5.5 branch.
Fixes #49236.
Built from https://develop.svn.wordpress.org/branches/5.5@48752


git-svn-id: https://core.svn.wordpress.org/branches/5.5@48514 1a063a9b-81f0-0310-95a4-ce76da25c4cd