Don't copy the functor object for each signal emission

The behavior was different in the variadic template code and in the C++98
code. The code without variadic template support was not copying the functor
object (e.g. a lambda) before calling it.
However, in the variadic template section, QtPrivate::FunctorCall::call
took the functor by value instead of by reference resulting in a copy.

QtPrivate::FunctorCall::call is a helper function for
QtPrivate::FunctionPointer::call which is only needed for variadic template
expension.

[ChangeLog][QtCore][QObject] If the compiler supports variadic templates,
no longer copy functor connected to a signal each time the signal is
emitted. Restoring the C++98 behavior.

Task-number: QTBUG-52542
Change-Id: I3ca20ef6910893b8a288e70af7de4c7b69502173
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>