Update for llvm API change.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@208984 91177308-0d34-0410-b5e6-96231b3b80d8
Tooling: use stderr, not stdout for missing command line messages

Matches other tooling output that use stderr.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@208988 91177308-0d34-0410-b5e6-96231b3b80d8