Revert "[Clang] Add a warning on invalid UTF-8 in comments."

Revert "[Clang] Add a warning on invalid UTF-8 in comments."

This reverts commit 4174f0ca618b467571b43cff12cbe4c4239670f8.

Also revert follow-up "[Clang] Fix invalid utf-8 detection"
This reverts commit bf45e27a676d87944f1f13d5f0d0f39935fc4010.

The second commit broke tests, see comments on
https://reviews.llvm.org/D129223, and it sounds like the first
commit isn't valid without the second one. So reverting both for now.