Activity: Clear comment cache when adding an activity comment with `bp_activity_add()`.

This commit fixes issues with the activity comment cache not being cleared
when a new activity comment is created using `bp_activity_add()`.

This notably occurs when posting a blog comment and when the activity stream
commenting for blog posts feature is enabled.  See `bp_blogs_record_comment()`.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8996 cdf35c40-ae34-48e0-9cc9-0c9da1808c22