[libunwind] [SEH] Handle ExceptionContinueExecution in forced unwinding

[mstorsjo]

Summary

This fixes the libcxxabi test force_unwind3.pass.cpp when run on native
Windows.

When unwinding past the main thread function into the system functions
that brought up the thread, we can hit functions whose personality
functions return ExceptionContinueExecution (instead of the regular
ExceptionContinueSearch). Interpret this as a signal to stop the
unwind.

Curiously, in this case, it does return ExceptionContinueSearch if
running within a debugger.

[phosek]

LGTM

libunwind/src/Unwind-seh.cpp

__libunwind_seh_personality(int version, _Unwind_Action state,
   ms_exc.ExceptionInformation[2] = state;
   DISPATCHER_CONTEXT *disp_ctx =
       __unw_seh_get_disp_ctx((unw_cursor_t *)context);
   EXCEPTION_DISPOSITION ms_act = disp_ctx->LanguageHandler(&ms_exc,
                                                            (PVOID)disp_ctx->EstablisherFrame,
                                                            disp_ctx->ContextRecord,
                                                            disp_ctx);
   switch (ms_act) {
+  case ExceptionContinueExecution: return _URC_END_OF_STACK;
   case ExceptionContinueSearch: return _URC_CONTINUE_UNWIND;
   case 4 /*ExceptionExecuteHandler*/:
     return phase2 ? _URC_INSTALL_CONTEXT : _URC_HANDLER_FOUND;
   default:
     return phase2 ? _URC_FATAL_PHASE2_ERROR : _URC_FATAL_PHASE1_ERROR;
   }
 }
 
if (frameInfo.handler != 0) {
       case _URC_INSTALL_CONTEXT:
         _LIBUNWIND_TRACE_UNWINDING("unwind_phase2_forced(ex_ojb=%p): "
                                    "personality returned "
                                    "_URC_INSTALL_CONTEXT",
                                    (void *)exception_object);
         // We may get control back if landing pad calls _Unwind_Resume().
         __unw_resume(&cursor2);
         break;
+      case _URC_END_OF_STACK:
+        _LIBUNWIND_TRACE_UNWINDING("unwind_phase2_forced(ex_ojb=%p): "
+                                   "personality returned "
+                                   "_URC_END_OF_STACK",
+                                   (void *)exception_object);
+        break;
       default:
         // Personality routine returned an unknown result code.
         _LIBUNWIND_TRACE_UNWINDING("unwind_phase2_forced(ex_ojb=%p): "
                                    "personality returned %d, "
                                    "_URC_FATAL_PHASE2_ERROR",
                                    (void *)exception_object, personalityResult);
         return _URC_FATAL_PHASE2_ERROR;
       }
+      if (personalityResult == _URC_END_OF_STACK)
+        break;
     }
   }
 
   // Call stop function one last time and tell it we've reached the end
   // of the stack.
   _LIBUNWIND_TRACE_UNWINDING("unwind_phase2_forced(ex_ojb=%p): calling stop "
                              "function with _UA_END_OF_STACK",
                              (void *)exception_object);