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

llvm-svn: 191618
diff --git a/lldb/source/Plugins/Process/Linux/ProcessMonitor.cpp b/lldb/source/Plugins/Process/Linux/ProcessMonitor.cpp
index d9d9093..5578748 100644
--- a/lldb/source/Plugins/Process/Linux/ProcessMonitor.cpp
+++ b/lldb/source/Plugins/Process/Linux/ProcessMonitor.cpp
@@ -1111,7 +1111,8 @@
         eDupStdoutFailed,
         eDupStderrFailed,
         eChdirFailed,
-        eExecFailed
+        eExecFailed,
+        eSetGidFailed
     };
 
     // Child process.
@@ -1122,7 +1123,8 @@
             exit(ePtraceFailed);
 
         // Do not inherit setgid powers.
-        setgid(getgid());
+        if (setgid(getgid()) != 0)
+            exit(eSetGidFailed);
 
         // Let us have our own process group.
         setpgid(0, 0);
@@ -1187,6 +1189,9 @@
             case eExecFailed:
                 args->m_error.SetErrorString("Child exec failed.");
                 break;
+            case eSetGidFailed:
+                args->m_error.SetErrorString("Child setgid failed.");
+                break;
             default:
                 args->m_error.SetErrorString("Child returned unknown exit status.");
                 break;