Code reorg in preparation for adding comdat types support.
* dwarf2read.c (struct die_reader_specs): New struct.
(locate_pdi_sibling): New arg buffer. All callers updated.
(load_partial_dies, read_partial_die): Ditto.
(read_8_bytes): Change return type to ULONGEST.
(read_comp_unit): Delete arg abfd. All callers updated.
(read_die_and_children_1): Delete args abfd, cu. New arg reader.
All callers updated.
(read_die_and_children, read_die_and_siblings): Ditto.
(read_full_die): Ditto. Move closer to callers.
(skip_one_die): New arg buffer. All callers updated.
(load_full_comp_unit): Change return type to void. All callers
updated.
(partial_read_comp_unit_head): New args buffer, buffer_size.
All callers updated.
(process_psymtab_comp_unit): New function, split out from
dwarf2_build_psymtabs_hard.
(dwarf2_build_psymtabs_hard): Call it.
(load_partial_comp_unit): Renamed from load_comp_unit.
All callers updated.
(skip_children): New arg buffer. All callers updated.
(init_cu_die_reader): New function.
(is_ref_attr): New function.
(dwarf2_get_ref_die_offset): Call it.
(alloc_one_comp_unit): New function.