clover: Add explicit virtual destructor to argument class

It is needed to destroy the v vector in scalar_argument
Fixes memory leaks on parameter set/bind.

v2: Drop redundant sclara_argument destructor

Signed-off-by: Jan Vesely <jan.vesely@rutgers.edu>
Reviewed-by: Francisco Jerez <currojerez@riseup.net>
(cherry picked from commit 2f1ad72a)