QVarLengthArray: use new assign() in op=(initializer_list)

DRY. Besides, assign() is optimized more.

Change-Id: I6c47ed99292d69f887dd187cdc7fef596e2e7d37
Reviewed-by: Dennis Oberst <dennis.oberst@qt.io>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

src/corelib/tools/qvarlengtharray.h

@@ -379,9 +379,7 @@ public:
 
     QVarLengthArray<T, Prealloc> &operator=(std::initializer_list<T> list)
     {
-        resize(qsizetype(list.size()));
-        std::copy(list.begin(), list.end(),
-                  QT_MAKE_CHECKED_ARRAY_ITERATOR(begin(), size()));
+        assign(list);
         return *this;
     }