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

This reverts commit 4174f0ca.

Also revert follow-up "[Clang] Fix invalid utf-8 detection"
This reverts commit bf45e27a.

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.