Change two functions to be methods on struct attribute

This changes dwarf2_get_ref_die_offset and
dwarf2_get_attr_constant_value to be methods on struct attribute.

gdb/ChangeLog
2020-03-26  Tom Tromey  <tom@tromey.com>

	* dwarf2/read.c (handle_data_member_location, dwarf2_add_field)
	(mark_common_block_symbol_computed, read_tag_string_type)
	(attr_to_dynamic_prop, read_subrange_type): Update.
	(dwarf2_get_ref_die_offset, dwarf2_get_attr_constant_value): Move
	to be methods on struct attribute.
	(skip_one_die, process_imported_unit_die, read_namespace_alias)
	(read_call_site_scope, partial_die_info::read)
	(partial_die_info::read, lookup_die_type, follow_die_ref):
	Update.
	* dwarf2/attribute.c (attribute::get_ref_die_offset): New method,
	from dwarf2_get_ref_die_offset.
	(attribute::constant_value): New method, from
	dwarf2_get_attr_constant_value.
	* dwarf2/attribute.h (struct attribute) <get_ref_die_offset>:
	Declare method.
	<constant_value>: New method.