Force a [NAN, NAN] range when the definite NAN property is set.

Force a [NAN, NAN] range when the definite NAN property is set.

Setting the definite NAN property should also force a [NAN, NAN]
range, otherwise we'd have two ways of representing a NAN: with the
endpoints or with the property.  In the ranger world we avoid at all
costs having more than one representation for a range.

In doing this, I removed the FRANGE_PROP_ACCESSOR macro, since it
looks like setting a property may have repercurssions in the range
itself, so it's best for the client to definte its own setter.

gcc/ChangeLog:

	* value-range-storage.cc (frange_storage_slot::get_frange): Use
	frange_nan.
	* value-range.cc (frange::set_nan): New.
	(frange_nan): Move to header file.
	(range_tests_nan): Adjust frange_nan callers to pass type.
	New test.
	* value-range.h (FRANGE_PROP_ACCESSOR): Remove.
	(frange_nan): New.