Switch the LLVM error reporting on the new diagnostic system.
The LLVM diagnostic are now wired-up in clang (since r200931),
thus the user experience will not be impacted by this change
anymore.

Related to <rdar://problem/15886697>


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201915 91177308-0d34-0410-b5e6-96231b3b80d8