Debugger[CDB]: Switch to thread 0 after DebugBreak().

Do not show artificial thread that is created by
DebugBreak() as it causes a switch to disassembly.
Forcibly discard the symbol group for each locals
update as the lazy creation mechanism does not trigger
on thread changes back and forth involving assembly,
which causes the symbol group to become stale.

src/plugins/debugger/cdb/cdbengine.cpp

@@ -1111,9 +1111,13 @@ void CdbEngine::updateLocals()
         watchHandler()->endCycle();
         return;
     }
-    // Watchers: Initial expand, get uninitialized and query
+    /* Watchers: Forcibly discard old symbol group as switching from
+     * thread 0/frame 0 -> thread 1/assembly -> thread 0/frame 0 will otherwise re-use it
+     * and cause errors as it seems to go 'stale' when switching threads.
+     * Initial expand, get uninitialized and query */
     QByteArray arguments;
     ByteArrayInputStream str(arguments);
+    str << "-D";
     // Pre-expand
     const QSet<QByteArray> expanded = watchHandler()->expandedINames();
     if (!expanded.isEmpty()) {
@@ -1428,7 +1432,9 @@ enum StopActionFlags
     StopShowExceptionMessageBox = 0x4,
     // Notify stop or just continue
     StopNotifyStop = 0x8,
-    StopIgnoreContinue = 0x10
+    StopIgnoreContinue = 0x10,
+    // Hit on break in artificial stop thread (created by DebugBreak()).
+    StopInArtificialThread = 0x20
 };
 
 unsigned CdbEngine::examineStopReason(const QByteArray &messageIn,
@@ -1483,8 +1489,12 @@ unsigned CdbEngine::examineStopReason(const QByteArray &messageIn,
             }
         }
         if (isDebuggerWinException(exception.exceptionCode)) {
+            unsigned rc = StopReportStatusMessage|StopNotifyStop;
+            // Detect interruption by DebugBreak() and force a switch to thread 0.
+            if (exception.function == "ntdll!DbgBreakPoint")
+                rc |= StopInArtificialThread;
             *message = msgInterrupted();
-            return StopReportStatusMessage|StopNotifyStop;
+            return rc;
         }
 #endif
         *exceptionBoxMessage = msgStoppedByException(description, QString::number(threadId));
@@ -1561,6 +1571,10 @@ void CdbEngine::handleSessionIdle(const QByteArray &messageBA)
             notifyInferiorSpontaneousStop();
         }
         // Start sequence to get all relevant data.
+        if (stopFlags & StopInArtificialThread) {
+            showMessage(tr("Switching to main thread..."), LogMisc);
+            postCommand("~0 s", 0);
+        }
         unsigned sequence = CommandListStack|CommandListThreads;
         if (debuggerCore()->isDockVisible(QLatin1String(Constants::DOCKWIDGET_REGISTER)))
             sequence |= CommandListRegisters;