Add reprocessing flag to struct attribute

Add reprocessing flag to struct attribute

Some forms require "reprocessing" -- a second pass to update their
value appropriately.  In this case, we'll set the unsigned value on
the attribute, and then later set it to the correct value.

To handle this, we introduce a reprocessing flag to attribute.  Then,
we manage this flag to ensure that setting and unsetting is done
properly.

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

	* dwarf2/read.c (read_cutu_die_from_dwo): Use OBSTACK_ZALLOC.
	(read_attribute_reprocess, read_attribute_value): Update.
	(read_attribute): Clear requires_reprocessing.
	* dwarf2/attribute.h (struct attribute) <as_unsigned_reprocess,
	form_requires_reprocessing>: New methods.
	<string_init>: Clear requires_reprocessing.
	<set_unsigned_reprocess>: New method.
	<name>: Shrink by one bit.
	<requires_reprocessing>: New member.
	* dwarf2/attribute.c (attribute::form_requires_reprocessing): New
	method.