Fix living QLibrary member after shutdown of QCoreApplication

LibResolv uses a QLibrary which is a QObject that must be deleted
if the QCoreApplication is being destroyed to release the underlying library.
A Q_GLOBAL_STATIC won't release any memory and is not able to
manually release it.

Pick-to: 5.15
Task-number: QTBUG-84234
Change-Id: I97fe5faca309e9c1e85435f602ad7f8c3f633b48
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/network/kernel/qhostinfo_unix.cpp

@@ -156,7 +156,25 @@ LibResolv::LibResolv()
         }
     }
 }
-Q_GLOBAL_STATIC(LibResolv, libResolv)
+
+LibResolv* libResolv()
+{
+    static LibResolv* theLibResolv = nullptr;
+    static QBasicMutex theMutex;
+
+    const QMutexLocker locker(&theMutex);
+    if (theLibResolv == nullptr) {
+        theLibResolv = new LibResolv();
+        Q_ASSERT(QCoreApplication::instance());
+        QObject::connect(QCoreApplication::instance(), &QCoreApplication::destroyed, [] {
+            const QMutexLocker locker(&theMutex);
+            delete theLibResolv;
+            theLibResolv = nullptr;
+        });
+    }
+
+    return theLibResolv;
+}
 
 static void resolveLibrary(LibResolvFeature f)
 {