Make the empty Q_ASSERT still check its argument for validity

With the side-effect that variables and functions in the argument are
now "used". This means we don't need to #ifndef functions only used in
assertions: the compiler will eliminate them by dead code elimination.
Due to the mandatory short-circuiting of the condition, no functions
will ever be called and this expands to no more code than before.

On the negative side, because we won't get warnings for unused variables
initialized outside the Q_ASSERT, non-inlineable calls will not be
elminated by dead code elimination, so they will remain in release code
without warnings.

Because of the expanded code now in Q_ASSERT, the Intel compiler's
optimizer gets thrown: it complains that the non-void function is
failing to return anything, so I had to add two return statements.

Change-Id: I1bb79c9637a2771ef1ec093f901b8d2443788bd6
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@digia.com>
Fix leftover doc bug from Qt 3 days.

Change-Id: Ic5499ba2e3b27dcc5b36d273c01f92d57b26ce3a
Reviewed-by: Robin Burchell <robin.burchell@viroteck.net>