Fix call to non-existing getTransactionCommentForSave() in PhabricatorInlineComment

Summary:
rPb48a22bf50a32895566e9aa66c1d7aff87a492b8 moved the call $this->getTransactionCommentForSave()->save() from within PhabricatorAuditInlineComment to the new (parent) class PhabricatorInlineComment.php, however the later does not define this function.

As the two child classes which extend PhabricatorInlineComment both implement getTransactionCommentForSave(), declare an abstract function in their parent class.

Test Plan: Carefully read the code.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25644