Before the string was simply swapped with the other string which can lead
to an unexpected behavior for xvalues. Now the destructor of the source is
called and it is default initialized.

foo = std::move(bar);

bar would now hold the value of foo.

Change-Id: Ibea3f18333a168634b7faf2fdaf9b5b52c82d5cc
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>