[CIDFontType2] Map characters missing in toUnicode to the private use area (bug 1028735 and issue 4881)

Fixes https://bugzilla.mozilla.org/show_bug.cgi?id=1028735.
Fixes #4881.

*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.)

**Edit:** Given that this PR basically reverts one aspect of `CIDFontType2` font parsing to the state prior to PR #4259, I'm not sure if this patch is the ideal way to fix these regressions.
Even though this PR does fix two regressions, there is obviously a risk that it can regress other PDF files (where we currently don't have sufficient test coverage).