Remove DW_SIGNATURE

Remove DW_SIGNATURE

This removes DW_SIGNATURE in favor of methods on struct attribute.  As
usual, the methods check the form, which DW_SIGNATURE did not do.

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

	* dwarf2/read.c (read_attribute_value, lookup_die_type)
	(dump_die_shallow, follow_die_sig, get_DW_AT_signature_type):
	Update.
	* dwarf2/attribute.h (struct attribute) <as_signature,
	set_signature>: New methods.
	(DW_SIGNATURE): Remove.