[lldb-mi] Remove use of dialog box

[xiaobai]

Summary

This really is only implemented on Windows, and it requires us to pull
in User32. This was only useful when debugging on lldb-mi on Windows, and there
doesn't seem to be a good reason why using a dialog box is better than what
exists for other platforms.

[jingham]

LGTM

tools/lldb-mi/MIDriverMain.cpp

 //                <0    = Program failed.
 //              -1      = Program failed reason not specified here, see MI log
 //              file.
 //              -1000   = Program failed did not initialize successfully.
 // Throws:  None.
 //--
 int main(int argc, char const *argv[]) {
 #if MICONFIG_DEBUG_SHOW_ATTACH_DBG_DLG
-#ifdef _WIN32
-  CMIUtilDebug::ShowDlgWaitForDbgAttach();
-#else
   CMIUtilDebug::WaitForDbgAttachInfinteLoop();
-#endif //  _WIN32
 #endif // MICONFIG_DEBUG_SHOW_ATTACH_DBG_DLG
 
   llvm::StringRef ToolName = argv[0];
   llvm::sys::PrintStackTraceOnErrorSignal(ToolName);
   llvm::PrettyStackTraceProgram X(argc, argv);
 
   // *** Order is important here ***
   bool bOk = DriverSystemInit();

tools/lldb-mi/MIUtilDebug.h

 
 //++
 //============================================================================
 // Details: MI debugging aid utility class.
 //--
 class CMIUtilDebug {
   // Statics:
 public:
-  static void ShowDlgWaitForDbgAttach();
   static void WaitForDbgAttachInfinteLoop();
 
   // Methods:
 public:
   /* ctor */ CMIUtilDebug();
 
   // Overrideable:
 public:

tools/lldb-mi/MIUtilDebug.cpp

 // Args:    None.
 // Return:  None.
 // Throws:  None.
 //--
 CMIUtilDebug::~CMIUtilDebug() {}
 
 //++
 //------------------------------------------------------------------------------------
-// Details: Show a dialog to the process/application halts. It gives the
-// opportunity to
-//          attach a debugger.
-// Type:    Static method.
-// Args:    None.
-// Return:  None.
-// Throws:  None.
-//--
-void CMIUtilDebug::ShowDlgWaitForDbgAttach() {
-  const CMIUtilString strCaption(CMIDriver::Instance().GetAppNameShort());
-#ifdef _WIN32
-  ::MessageBoxA(NULL, "Attach your debugger now", strCaption.c_str(), MB_OK);
-#else
-// ToDo: Implement other platform version of an Ok to continue dialog box
-#endif // _WIN32
-}
-
-//++
-//------------------------------------------------------------------------------------
 // Details: Temporarily stall the process/application to give the programmer the
 //          opportunity to attach a debugger. How to use: Put a break in the
 //          programmer
 //          where you want to visit, run the application then attach your
 //          debugger to the
 //          application. Hit the debugger's pause button and the debugger should
 //          should
 //          show this loop. Change the i variable value to break out of the loop