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

This reverts commit cc309721 because it
breaks the following tests on GreenDragon:

  TestDataFormatterObjCCF.py
  TestDataFormatterObjCExpr.py
  TestDataFormatterObjCKVO.py
  TestDataFormatterObjCNSBundle.py
  TestDataFormatterObjCNSData.py
  TestDataFormatterObjCNSError.py
  TestDataFormatterObjCNSNumber.py
  TestDataFormatterObjCNSURL.py
  TestDataFormatterObjCPlain.py
  TestDataFormatterObjNSException.py

https://green.lab.llvm.org/green/view/LLDB/job/lldb-cmake/45288/