Remove DW_SND

Remove DW_SND

This removes DW_SND in favor of accessors on struct attribute.
These accessors check that the form is appropriate.

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

	* dwarf2/read.c (get_alignment, read_array_order)
	(read_attribute_value, dwarf2_const_value_attr)
	(dump_die_shallow, dwarf2_fetch_constant_bytes): Update.
	* dwarf2/attribute.h (struct attribute) <as_signed, set_signed>:
	New methods.
	(DW_SND): Remove.