QFlags: mark as Q_PRIMITIVE_TYPE

QFlags: mark as Q_PRIMITIVE_TYPE

I originally tried to put Q_DECLARE_TYPEINFOs into
Q_DECLARE_OPERATORS_FOR_FLAGS, to declare not only
the flags type, but also the underlying enum as
primitive, but too many users (arguably correctly)
used Q_DECLARE_OPERATORS_FOR_FLAGS at (non-global)
namespace scope where QTypeInfo would have been
specialised in the wrong namespace.

So specialise QTypeInfo for QFlags<T> only.

Change-Id: I4af6e29aefbd9460a3d2bc6405f03cdf6b1096bc
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>