Stop moving "Cc" addresses to "To" when building mail targets

Summary: Fixes T10539. When building mail targets, we fail to preserve whether a recipient was originally "To" or "Cc", and just move everyone to "To".

Test Plan:
Added a comment to a task with a "To" user and a "Cc" user, with metamta.placeholder-to-recipient set and metamta.one-mail-per-recipient set.

Got mail with me Cc'd as the Cc'd user:

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10539

Differential Revision: https://secure.phabricator.com/D15465