VR: add more virtual dtors

VR: add more virtual dtors

Add 2 virtual destructors in order to address:

gcc/alloc-pool.h:522:5: warning: destructor called on non-final 'value_range_equiv' that has virtual functions but non-virtual destructor [-Wdelete-non-abstract-non-virtual-dtor]
gcc/ggc.h:166:3: warning: destructor called on non-final 'int_range<1>' that has virtual functions but non-virtual destructor [-Wdelete-non-abstract-non-virtual-dtor]

gcc/ChangeLog:

	* value-range-equiv.h (class value_range_equiv): Add virtual
	  destructor.
	* value-range.h: Likewise.