Remove last cleanups from stabsread.c

Remove last cleanups from stabsread.c

This removes the last cleanups from stabsread.c.  Similar code in
dwarf2read.c was C++-ified, but considering that stabs are deprecated,
it seemed simpler to just change these allocations to use an obstack
and leave the data structures in place.

This patch renames field_info to stabs_field_info -- adding a
constructor here provoked a bug due to the resulting ODR violation.

2019-03-06  Tom Tromey  <tom@tromey.com>

	* stabsread.c (struct stabs_field_info): Rename from field_info.
	<list, fnlist>: Add initializers.
	<obstack>: New member.
	(read_member_functions, read_struct_fields, read_baseclasses):
	Allocate on obstack.  Don't use cleanups.
	(read_one_struct_field, read_member_functions, read_struct_fields)
	(read_baseclasses, read_tilde_fields, attach_fn_fields_to_type)
	(attach_fields_to_type, read_cpp_abbrev, read_member_functions)
	(read_struct_type): Update.