Avoid using DW_* macros in dwarf2/attribute.c

Avoid using DW_* macros in dwarf2/attribute.c

There's no need to use the DW_* accessor macros in dwarf2/attribute.c,
and this is a necessary step toward our goal of removing them
entirely.

gdb/ChangeLog
2020-09-29  Tom Tromey  <tom@tromey.com>

	* dwarf2/attribute.c (attribute::address): Don't use DW_UNSND or
	DW_ADDR.
	(attribute::string): Don't use DW_STRING.
	(attribute::get_ref_die_offset): Don't use DW_UNSND.
	(attribute::constant_value): Don't use DW_UNSND or DW_SND.