Debugger: Distinguish stdout and stderr in LLDB

At least theoretically, on our side. Practically the output
seems to arrive all with ldb.SBProcess.eBroadcastBitSTDOUT
from LLDB.

Change-Id: I61cfb1e01193c57e06aebeaa8defa11809e24c50
Reviewed-by: Eike Ziller <eike.ziller@digia.com>