[CIDFontType2] Map characters missing in toUnicode to the private use area (bug 1028735)

Fixes https://bugzilla.mozilla.org/show_bug.cgi?id=1028735.

*Please note:* The PDF file linked in the bug has been changed on the server, and the new file isn't affected by this issue, however I still think it makes sense to fix this since it's a regression.
Also, the test-case included here is *obviously* created from the previous (broken) version of the PDF file.

(Depends on PR #5014.)