Replace init_cutu_and_read_dies with a class

init_cutu_and_read_dies takes a callback function, which I've always
found somewhat difficult to follow.  This patch replaces this function
with a class, and changes the callers to use it.  In some cases this
allows for the removal of a helper struct and helper function as well.

gdb/ChangeLog
2020-01-19  Tom Tromey  <tom@tromey.com>

	* dwarf2read.c (abbrev_table_up): Move typedef earlier.
	(die_reader_func_ftype): Remove.
	(cutu_reader): New class.
	(dw2_get_file_names_reader): Remove "data" parameter.
	(dw2_get_file_names): Use cutu_reader.
	(create_debug_type_hash_table): Update.
	(read_cutu_die_from_dwo): Update comment.
	(lookup_dwo_unit): Add dwo_name parameter.
	(cutu_reader::init_tu_and_read_dwo_dies): Now a method.  Remove
	die_reader_func_ftype and data parameters.
	(cutu_reader::cutu_reader): Rename from init_cutu_and_read_dies.
	Remove die_reader_func_ftype and data parameters.
	(~cutu_reader): New; from init_cutu_and_read_dies.
	(cutu_reader::cutu_reader): Rename from
	init_cutu_and_read_dies_no_follow.  Remove die_reader_func_ftype
	and data parameters.
	(init_cutu_and_read_dies_simple): Remove.
	(struct process_psymtab_comp_unit_data): Remove.
	(process_psymtab_comp_unit_reader): Remove data parameter; add
	want_partial_unit and pretend_language parameters.
	(process_psymtab_comp_unit): Use cutu_reader.
	(build_type_psymtabs_reader): Remove data parameter.
	(build_type_psymtabs_1): Use cutu_reader.
	(process_skeletonless_type_unit): Likewise.
	(load_partial_comp_unit_reader): Remove.
	(load_partial_comp_unit): Use cutu_reader.
	(load_full_comp_unit_reader): Remove.
	(load_full_comp_unit): Use cutu_reader.
	(struct create_dwo_cu_data): Remove.
	(create_dwo_cu_reader): Remove datap parameter; add dwo_file and
	dwo_unit parameters.
	(create_cus_hash_table): Use cutu_reader.
	(struct dwarf2_read_addr_index_data): Remove.
	(dwarf2_read_addr_index_reader): Remove.
	(dwarf2_read_addr_index): Use cutu_reader.
	(read_signatured_type_reader): Remove.
	(read_signatured_type): Use cutu_reader.

Change-Id: I4ef2f29e73108ce94bfe97799f8f638ed272212d