Fix crash in libclang if code completion is used with unknown flags.

Use CaptureDroppedDiagnostics to make sure that there is a diagnostic
client installed when warning flags are parsed.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@186582 91177308-0d34-0410-b5e6-96231b3b80d8