Use new QBA::assign in QCoreApplication::applicationFilePath()

QByteArray::assign() re-uses existing unshared capacity(), if any, and
is therefore potentially more efficient than '= QByteArray(~)' (and
never slower).

Task-number: QTBUG-106201
Pick-to: 6.6
Change-Id: Ieeb254afd94e26f1b425795feb53c59ebb2322c6
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>
Reviewed-by: Marc Mutz <marc.mutz@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/kernel/qcoreapplication.cpp

@@ -2478,7 +2478,7 @@ QString QCoreApplication::applicationFilePath()
         if (procName != d->argv[0]) {
             // clear the cache if the procname changes, so we reprocess it.
             QCoreApplicationPrivate::clearApplicationFilePath();
-            procName = QByteArray(d->argv[0]);
+            procName.assign(d->argv[0]);
         }
     }