If setgid fails for any reason, exit lldb.

scan-build was complaining about:
The return value from the call to 'setgid' is not checked. If an error occurs in 'setgid', the following code may execute with unexpected privileges






git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@191618 91177308-0d34-0410-b5e6-96231b3b80d8