Make vrange an abstract class.

Make vrange an abstract class.

Explicitly make vrange an abstract class.  This involves fleshing out
the unsupported_range overrides which we were inheriting by default
from vrange.

gcc/ChangeLog:

	* value-range.cc (unsupported_range::accept): Move down.
	(vrange::contains_p):  Rename to...
	(unsupported_range::contains_p): ...this.
	(vrange::singleton_p): Rename to...
	(unsupported_range::singleton_p): ...this.
	(vrange::set): Rename to...
	(unsupported_range::set): ...this.
	(vrange::type): Rename to...
	(unsupported_range::type): ...this.
	(vrange::supports_type_p): Rename to...
	(unsupported_range::supports_type_p): ...this.
	(vrange::set_undefined): Rename to...
	(unsupported_range::set_undefined): ...this.
	(vrange::set_varying): Rename to...
	(unsupported_range::set_varying): ...this.
	(vrange::union_): Rename to...
	(unsupported_range::union_): ...this.
	(vrange::intersect): Rename to...
	(unsupported_range::intersect): ...this.
	(vrange::zero_p): Rename to...
	(unsupported_range::zero_p): ...this.
	(vrange::nonzero_p): Rename to...
	(unsupported_range::nonzero_p): ...this.
	(vrange::set_nonzero): Rename to...
	(unsupported_range::set_nonzero): ...this.
	(vrange::set_zero): Rename to...
	(unsupported_range::set_zero): ...this.
	(vrange::set_nonnegative): Rename to...
	(unsupported_range::set_nonnegative): ...this.
	(vrange::fits_p): Rename to...
	(unsupported_range::fits_p): ...this.
	(unsupported_range::operator=): New.
	(frange::fits_p): New.
	* value-range.h (class vrange): Make an abstract class.
	(class unsupported_range): Declare override methods.