Flush cache for newly created term in `_split_shared_term()`.

The term itself does not have any cached values yet, but in some cases the new
term's taxonomy may need its cached hierarchy to be refreshed as a result of
the term splitting.

Props jorbin.
See #30335.

git-svn-id: https://develop.svn.wordpress.org/trunk@30347 602fd350-edb4-49c9-b593-d223f7449a82