Fix comment in dwarf2/attribute.h

I noticed that a comment in dwarf2/attribute.h still referred to
dwarf2_get_attr_constant_value.  However, this is now a method on
struct attribute.

gdb/ChangeLog
2020-03-28  Tom Tromey  <tom@tromey.com>

	* dwarf2/attribute.h (struct attribute) <form_is_constant>: Update
	comment.