linux process: silence GCC switch coverage warning

Add missing entry for eExecMessage message type to silence GCC switch coverage
warning.

llvm-svn: 213470

lldb/source/Plugins/Process/Linux/ProcessMonitor.cpp

@@ -1844,6 +1844,8 @@ ProcessMonitor::StopThread(lldb::tid_t tid)
 
         switch (message.GetKind())
         {
+            case ProcessMessage::eExecMessage:
+                llvm_unreachable("unexpected message");
             case ProcessMessage::eAttachMessage:
             case ProcessMessage::eInvalidMessage:
                 break;