Fix warning when building objects with a Q_OBJECT macro

Do not trigger a self-assign warning when running into code containing
a Q_OBJECT macro. Currently this happens a lot e.g. when using clang
to build code using Qt.

Change-Id: I68995624b18406f337318599e463f36f87486e66
(cherry-picked from qtbase commit 1416f4c595d6078c08f93483695e0b64c7fbb2a7)
Reviewed-by: Milian Wolff <milian.wolff@kdab.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>