Remove DW_ADDR

Remove DW_ADDR

This removes DW_ADDR in favor of accessor methods.

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

	* dwarf2/read.c (read_attribute_reprocess, read_attribute_value)
	(dwarf2_const_value_attr, dump_die_shallow)
	(dwarf2_fetch_constant_bytes): Update.
	* dwarf2/attribute.h (struct attribute) <form_is_ref>: Update
	comment.
	<set_address>: New method.
	(DW_ADDR): Remove.
	* dwarf2/attribute.c (attribute::form_is_ref): Update comment.
	(attribute::as_string, attribute::as_address): Add assert.