Remove dwarf2_per_bfd::all_type_units

Remove dwarf2_per_bfd::all_type_units

I don't think there is any deep reason to separate CUs and TUs in
dwarf2_per_bfd.  This patch removes all_type_units and unifies these
two containers.  Some minor tweaks are needed to the index writers,
because both forms of index keep CUs and TUs separate;

Regression tested on x86-63 Fedora 32.

gdb/ChangeLog
2021-04-30  Tom Tromey  <tom@tromey.com>

	* dwarf2/read.h (struct tu_stats) <nr_tus>: New member.
	(struct dwarf2_per_bfd) <get_cutu, get_tu>: Remove
	<get_cu>: Now inline.
	<all_type_units>: Remove.
	* dwarf2/read.c (dwarf2_per_bfd::~dwarf2_per_bfd): Update.
	(dwarf2_per_bfd::get_cutu, dwarf2_per_bfd::get_cu)
	(dwarf2_per_bfd::get_tu): Remove.
	(dwarf2_per_bfd::allocate_signatured_type): Update nr_tus.
	(create_signatured_type_table_from_index)
	(create_signatured_type_table_from_debug_names)
	(dw2_symtab_iter_next, dwarf2_base_index_functions::print_stats)
	(dwarf2_base_index_functions::expand_all_symtabs)
	(dw2_expand_marked_cus, dw_expand_symtabs_matching_file_matcher)
	(dwarf2_base_index_functions::map_symbol_filenames)
	(dw2_debug_names_iterator::next, dwarf2_initialize_objfile)
	(add_signatured_type_cu_to_table, create_all_type_units)
	(add_type_unit, build_type_psymtabs_1, print_tu_stats)
	(create_all_comp_units): Update.
	* dwarf2/index-write.c (check_dwarf64_offsets, write_gdbindex)
	(write_debug_names): Update.