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