Calling SBDebugger::CeeateTarget being called on multiple threads was crashing LLDB.

I found the race condition in:

ScriptInterpreter *CommandInterpreter::GetScriptInterpreter(bool can_create);

More than one "ScriptInterpreter *" was being returned due to the race which caused any clients with the first one to now be pointing to freed memory and we would quickly crash.

Added a test to catch this so we don't regress.

<rdar://problem/28356584> 

llvm-svn: 289169