QVarLengthArray: use assign() for the iterator,iterator ctor

Let's avoid std::back_inserter for non-complex types, which generates a
huge amount of unnecessary code. The implementation of assign() is far
more modern.

Change-Id: Ia8b9c90336af2bebd49ffffd0d69ace761c6aa59
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
Reviewed-by: Marc Mutz <marc.mutz@qt.io>
(cherry picked from commit 96dfe0373c80964ae9de7955a06f5c7696961f12)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/tools/qvarlengtharray.h

@@ -360,8 +360,7 @@ public:
     inline QVarLengthArray(InputIterator first, InputIterator last)
         : QVarLengthArray()
     {
-        QtPrivate::reserveIfForwardIterator(this, first, last);
-        std::copy(first, last, std::back_inserter(*this));
+        assign(first, last);
     }
 
     inline ~QVarLengthArray()