Clang: Ensure end command is send before waiting for process finish

This reverts

    CmbIpc: Remove workaround
    287a263083

since the bug still exists. However, we use waitForBytesWritten()
instead of QCoreApplication::processEvents() since this seems to be fine
(QAbstractSocket::waitForBytesWritten is not called on Windows).

The bug was:
 1. Open a bigger project, like qtcreator.pro.
 2. After 2 seconds, quit it.
 ==> The Qt Creator window is gone but the Qt Creator process waits
     until the clangbackend process is finished.

Change-Id: I25c475d2a6db94a1c3a8d827a41f8c0b2624eaea
Reviewed-by: Marco Bubke <marco.bubke@theqtcompany.com>

src/libs/clangbackendipc/connectionclient.cpp

@@ -95,10 +95,17 @@ bool ConnectionClient::isConnected() const
     return localSocket.state() == QLocalSocket::ConnectedState;
 }
 
+void ConnectionClient::ensureCommandIsWritten()
+{
+    while (localSocket.bytesToWrite() > 0)
+        localSocket.waitForBytesWritten();
+}
+
 void ConnectionClient::sendEndCommand()
 {
     serverProxy_.end();
     localSocket.flush();
+    ensureCommandIsWritten();
 }
 
 void ConnectionClient::resetProcessAliveTimer()