Remove DW_UNSND

This removes DW_UNSND, replacing uses with either as_unsigned or
constant_value, depending primarily on whether or not the form is
already known to be appropriate.

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

	* dwarf2/read.c (lookup_dwo_id, get_type_unit_group)
	(read_file_scope, dwarf2_get_pc_bounds)
	(dwarf2_record_block_ranges, dwarf2_add_field, get_alignment)
	(read_structure_type, handle_struct_member_die)
	(read_enumeration_type, read_array_type, read_set_type)
	(read_tag_pointer_type, read_tag_reference_type)
	(read_subroutine_type, read_base_type, read_subrange_type)
	(read_full_die_1, partial_die_info::read)
	(partial_die_info::read, by, new_symbol)
	(dwarf2_const_value_data, dwarf2_const_value_attr)
	(dump_die_shallow, dwarf2_fetch_constant_bytes)
	(prepare_one_comp_unit): Update.
	* dwarf2/attribute.h (DW_UNSND): Remove.