const_tree conversion of vrange::supports_*

const_tree conversion of vrange::supports_*

Make all vrange::supports_*_p methods const_tree as they can end up
being called from functions that are const_tree.

Tested on x86-64 Linux.

gcc/ChangeLog:

	* value-range.cc (vrange::supports_type_p): Use const_tree.
	(irange::supports_type_p): Same.
	(frange::supports_type_p): Same.
	* value-range.h (Value_Range::supports_type_p): Same.
	(irange::supports_p): Same.