QCoreApplication: mark instance() as noexcept

QEventLoopLocker's own noexcept depends on this.

Task-number: QTBUG-114793
Pick-to: 6.6
Change-Id: If1ba3aaa2f0bbb6334d52dd44db84d138890f4bb
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/kernel/qcoreapplication.h

@@ -88,7 +88,7 @@ public:
     static void setSetuidAllowed(bool allow);
     static bool isSetuidAllowed();
 
-    static QCoreApplication *instance() { return self; }
+    static QCoreApplication *instance() noexcept { return self; }
 
 #ifndef QT_NO_QOBJECT
     static int exec();