No checks on placeholders for XLIFF files

[Delphine]

(follow-up from conversation with flod and Matjaz)

Currently there are no checks on placeholders for iOS products in Pontoon.

Talking with Matjaz, it seems that we recently had a similar issue with Android: https://github.com/mozilla/pontoon/issues/2323

Indeed, this should go to compare-locales, but since it lacks support for XLIFF whatsoever, the lift is likely to be quite heavy. The first step we identified was filing this issue here.

The next step will be to research how big of an effort would it be to do the implementation properly (in c-l) vs. creating the checks first in Pontoon directly.