Use (new) QStringIterator(QStringView) to avoid casts

This is the only one in QtBase that casts to be able to use QStringIterator.

Amends d40dcee642.

Change-Id: I709e0285717e8d17feab9ab0020c8d0aed0dbe36
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>

src/corelib/tools/qstring.cpp

@@ -1596,7 +1596,7 @@ int QString::toUcs4_helper(const ushort *uc, int length, uint *out)
 {
     int count = 0;
 
-    QStringIterator i(reinterpret_cast<const QChar *>(uc), reinterpret_cast<const QChar *>(uc + length));
+    QStringIterator i(QStringView(uc, length));
     while (i.hasNext())
         out[count++] = i.next();